*: Fixed some linter check errors

[hsqlu]

Signed-off-by: Qiannan Lyu lvqiannan@gmail.com

What problem does this PR solve?

#22979 Fixed some linter check errors.

Problem Summary:

Before:

➜  tidb git:(master) make check-static                                                                                                 
tools/bin/golangci-lint run -v --disable-all --deadline=3m \
          --enable=misspell \                                                                                                          
          --enable=ineffassign \
          --enable=typecheck \
          --enable=varcheck \                                                                                                          
          --enable=unused \         
          --enable=structcheck \
          --enable=deadcode \                                                                                                          
          --enable=gosimple \       
          $(go list ./...| grep -vE "cmd|github.com\/pingcap\/tidb\/tests" | sed 's|github.com/pingcap/tidb/||')
INFO [config_reader] Config search paths: [./ /Users/qiannan/go/src/github.com/pingcap/tidb/bindinfo /Users/qiannan/go/src/github.com/pingcap/tidb /Users/qiannan/go/src/github.com/pingcap /Users/qiannan/go/src/github.com /Users/qiannan/go/src /Users/qiannan/go /Users/qi
annan /Users /]                                          
INFO [lintersdb] Active 8 linters: [deadcode gosimple ineffassign misspell structcheck typecheck unused varcheck] 
INFO [loader] Go packages loading at mode 575 (compiled_files|files|imports|types_sizes|deps|exports_file|name) took 2.154018256s 
INFO [runner/filename_unadjuster] Pre-built 0 adjustments in 217.231129ms 
INFO [linters context/goanalysis] analyzers took 0s with no stages 
INFO [runner] Issues before processing: 60, after processing: 17                                                                       
INFO [runner] Processors filtering stat (out/in): cgo: 60/60, filename_unadjuster: 60/60, skip_files: 60/60, exclude: 60/60, nolint: 17/60, max_same_issues: 17/17, severity-rules: 17/17, path_prettifier: 60/60, skip_dirs: 60/60, autogenerated_exclude: 60/60, identifier_
marker: 60/60, diff: 17/17, max_per_file_from_linter: 17/17, exclude-rules: 60/60, uniq_by_line: 17/17, source_code: 17/17, path_shortener: 17/17, path_prefixer: 17/17, sort_results: 17/17, max_from_linter: 17/17 
INFO [runner] processing took 81.31546ms with stages: nolint: 75.43776ms, source_code: 1.546283ms, autogenerated_exclude: 1.459192ms, path_prettifier: 1.356503ms, exclude-rules: 856.416µs, identifier_marker: 513.612µs, skip_dirs: 90.122µs, max_same_issues: 12.751µs, pat
h_shortener: 10.932µs, uniq_by_line: 9.566µs, max_from_linter: 6.41µs, cgo: 6.264µs, filename_unadjuster: 4.246µs, max_per_file_from_linter: 3.713µs, diff: 338ns, severity-rules: 326ns, sort_results: 323ns, skip_files: 294ns, exclude: 235ns, path_prefixer: 174ns 
INFO [runner] linters took 369.573059ms with stages: goanalysis_metalinter: 287.965682ms 
executor/builder.go:2794:93: `prefered` is a misspelling of `preferred` (misspell) 
        // When isPartition is set, it means the union rewriting is done, so a partition reader is prefered.
                                                                                                   ^
executor/builder.go:3023:93: `prefered` is a misspelling of `preferred` (misspell)
        // When isPartition is set, it means the union rewriting is done, so a partition reader is prefered.
                                                                                                   ^
sessionctx/variable/sysvar_test.go:116:17: ineffectual assignment to err (ineffassign)
        normalizedVal, err := sysVar.Validate(vars, "oN", ScopeSession)
                       ^
sessionctx/variable/sysvar_test.go:118:17: ineffectual assignment to err (ineffassign)
        normalizedVal, err = sysVar.Validate(vars, "0", ScopeSession)
                       ^
planner/core/preprocess.go:132:2: `initedLastSnapshotTS` is unused (structcheck)
        initedLastSnapshotTS bool
        ^
planner/core/logical_plan_builder.go:3637:2: S1023: redundant `return` statement (gosimple)
        return
        ^
structure/type.go:51:23: func `(*TxStructure).encodeHashMetaKey` is unused (unused)
func (t *TxStructure) encodeHashMetaKey(key []byte) kv.Key {
                      ^
statistics/handle/handle_test.go:872:26: func `(*testStatsSuite).checkForGlobalStatsWithOpts` is unused (unused)
func (s *testStatsSuite) checkForGlobalStatsWithOpts(c *C, tk *testkit.TestKit, p string, topn, buckets int) {
                         ^
statistics/handle/handle_test.go:849:26: func `(*testStatsSuite).prepareForGlobalStatsWithOpts` is unused (unused)
func (s *testStatsSuite) prepareForGlobalStatsWithOpts(c *C, tk *testkit.TestKit, tblName, dbName string) {
                         ^
statistics/handle/handle_test.go:890:7: type `opt` is unused (unused)
        type opt struct {
             ^
server/tidb_test.go:1490:2: ineffectual assignment to cancel (ineffassign)
        cancel = runWorkload(11, 20)
        ^
server/tidb_test.go:1508:2: ineffectual assignment to cancel (ineffassign)
        cancel = runWorkload(11, 20)
        ^
ddl/column_type_change_test.go:1767:14: S1039: unnecessary use of fmt.Sprintf (gosimple)
        tk.MustExec(fmt.Sprintf("set time_zone = 'UTC'"))
                    ^
executor/distsql_test.go:39:6: func `checkGoroutineExists` is unused (unused)
func checkGoroutineExists(keyword string) bool {
     ^
executor/analyze_test.go:602:22: func `(*testSuite1).testAnalyzeIncremental` is unused (unused)
func (s *testSuite1) testAnalyzeIncremental(tk *testkit.TestKit, c *C) {
                     ^
executor/memory_test.go:107:26: func `(*testMemoryLeak).readMem` is unused (unused)
func (s *testMemoryLeak) readMem() (allocated, heapInUse uint64) {
                         ^
executor/memory_test.go:113:26: func `(*testMemoryLeak).memDiff` is unused (unused)
func (s *testMemoryLeak) memDiff(m1, m2 uint64) uint64 {
                         ^
INFO File cache stats: 11 entries of total size 771.9KiB 
INFO Memory: 29 samples, avg is 109.7MB, max is 208.3MB 
INFO Execution took 2.763390788s                  
make: *** [check-static] Error 1

After:

➜  tidb git: make check-static 
tools/bin/golangci-lint run -v --disable-all --deadline=3m \
          --enable=misspell \
          --enable=ineffassign \
          --enable=typecheck \
          --enable=varcheck \
          --enable=unused \
          --enable=structcheck \
          --enable=deadcode \
          --enable=gosimple \
          $(go list ./...| grep -vE "cmd|github.com\/pingcap\/tidb\/tests" | sed 's|github.com/pingcap/tidb/||')
INFO [config_reader] Config search paths: [./ /Users/qiannan/go/src/github.com/pingcap/tidb/bindinfo /Users/qiannan/go/src/github.com/pingcap/tidb /Users/qiannan/go/src/github.com/pingcap /Users/qiannan/go/src/github.com /Users/qiannan/go/src /Users/qiannan/go /Users/qiannan /Users /] 
INFO [lintersdb] Active 8 linters: [deadcode gosimple ineffassign misspell structcheck typecheck unused varcheck] 
INFO [loader] Go packages loading at mode 575 (exports_file|files|imports|types_sizes|compiled_files|deps|name) took 2.149255847s 
INFO [runner/filename_unadjuster] Pre-built 0 adjustments in 206.892029ms 
INFO [linters context/goanalysis] analyzers took 0s with no stages 
INFO [runner] Issues before processing: 51, after processing: 1 
INFO [runner] Processors filtering stat (out/in): diff: 1/1, max_same_issues: 1/1, severity-rules: 1/1, autogenerated_exclude: 51/51, identifier_marker: 51/51, exclude-rules: 51/51, uniq_by_line: 1/1, source_code: 1/1, skip_dirs: 51/51, exclude: 51/51, path_shortener: 1/1, path_prefixer: 1/1, sort_results: 1/1, max_per_file_from_linter: 1/1, max_from_linter: 1/1, cgo: 51/51, filename_unadjuster: 51/51, path_prettifier: 51/51, skip_files: 51/51, nolint: 1/51 
INFO [runner] processing took 68.779247ms with stages: nolint: 65.77031ms, path_prettifier: 916.672µs, autogenerated_exclude: 891.398µs, exclude-rules: 611.232µs, identifier_marker: 408.53µs, source_code: 98.749µs, skip_dirs: 61.562µs, cgo: 6.973µs, filename_unadjuster: 5.893µs, uniq_by_line: 1.961µs, max_same_issues: 1.736µs, path_shortener: 1.256µs, max_from_linter: 874ns, max_per_file_from_linter: 544ns, skip_files: 411ns, diff: 345ns, exclude: 261ns, sort_results: 213ns, severity-rules: 204ns, path_prefixer: 123ns 
INFO [runner] linters took 342.291996ms with stages: goanalysis_metalinter: 273.374234ms 
planner/core/preprocess.go:133:2: `initedLastSnapshotTS` is unused (structcheck)
        initedLastSnapshotTS bool
        ^
INFO File cache stats: 1 entries of total size 45.9KiB 
INFO Memory: 29 samples, avg is 109.4MB, max is 207.5MB 
INFO Execution took 2.713330317s                  
make: *** [check-static] Error 1

Release note

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[ti-chi-bot]

@hsqlu: PR needs rebase.

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