fix: remove HEADER_LENGTH in decode because of discard

[Lvnszn]

What this PR does:
remove HEADER_LENGTH in decode because of discard
Which issue(s) this PR fixes:
Fixes #

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• All ut passed (run 'go test ./...' in project root)
• After go-fmt ed , run 'go fmt project' using goland.
• Golangci-lint passed, run 'sudo golangci-lint run' in project root.
• After import formatted, (using imports-formatter to run 'imports-formatter .' in project root, to format your import blocks, mentioned in CONTRIBUTING.md above)
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• All integration test passed. You can run integration test locally (with docker env). Clone our dubbo-go-samples project and replace the go.mod to your dubbo-go, and run 'sudo sh start_integration_test.sh' at root of samples project root. (M1 Slice is not Support)

[codecov-commenter]

Codecov Report

Merging #1831 (9e28951) into 3.0 (605764e) will decrease coverage by 0.03%.
The diff coverage is 0.00%.

❗ Current head 9e28951 differs from pull request most recent head 51d9749. Consider uploading reports for the commit 51d9749 to get more accurate results

@@            Coverage Diff             @@
##              3.0    #1831      +/-   ##
==========================================
- Coverage   46.76%   46.73%   -0.04%     
==========================================
  Files         298      298              
  Lines       17178    17178              
==========================================
- Hits         8034     8028       -6     
- Misses       8291     8296       +5     
- Partials      853      854       +1     

Impacted Files	Coverage Δ
protocol/dubbo/impl/codec.go	41.66% <0.00%> (ø)
filter/metrics/filter.go	85.00% <0.00%> (-15.00%)	⬇️
metrics/prometheus/reporter.go	26.85% <0.00%> (-1.72%)	⬇️

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