Make linter enable `unused`

[nolouch]

Development Task

ref #22979

check the package like:

➜  tidb git:(master) ✗ cd ddl
➜  ddl git:(master) ✗ go list ./...| grep -vE "cmd|github.com\/pingcap\/tidb\/tests" |  sed 's|github.com/pingcap/tidb/||'
ddl
ddl/failtest
ddl/placement
ddl/testutil
ddl/util
➜ tidb git:(master) ✗ tools/bin/golangci-lint run -v --disable-all --deadline=3m --enable=misspell  --enable=ineffassign --enable=unused ddl ddl/failtest ddl/placement ddl/testutil ddl/util
ddl/error.go:56:2: var `errUnknownTypeLength` is unused (unused)
ddl/column_type_change_test.go:43:2: field `dbInfo` is unused (unused)
INFO File cache stats: 0 entries of total size 0B

pacakge tasklist:

• bindinfo
• config
• ddl
• distsql
• domain
• errno
• executor
• expression
• infoschema
• kv
• lock
• meta
• metrics
• owner
• planner
• plugin
• privilege
• server
• session
• sessionctx
• statistics
• store
• structure
• table
• tablecodec
• telemetry
• tidb-server
• type