refactor: remove obsolete //+build comments

[stringsbuilder]

The plusbuild analyzer suggests a fix to remove obsolete build tags of the form:

    //+build linux,amd64

in files that also contain a Go 1.18-style tag such as:

    //go:build linux && amd64

(It does not check that the old and new tags are consistent;
that is the job of the 'buildtag' analyzer in the vet suite.)

[CLAassistant]

All committers have signed the CLA.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 19.98%. Comparing base (ef2bc66) to head (343d70e).
⚠️ Report is 39 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2450      +/-   ##
==========================================
+ Coverage   19.81%   19.98%   +0.16%     
==========================================
  Files        1090     1103      +13     
  Lines       73700    74605     +905     
==========================================
+ Hits        14604    14910     +306     
- Misses      57764    58336     +572     
- Partials     1332     1359      +27     

Files with missing lines	Coverage Δ
pkg/backtest/assets_dummy.go	0.00% <ø> (ø)
pkg/fixedpoint/div128.go	0.00% <ø> (ø)
pkg/util/trylock_18.go	0.00% <ø> (ø)

... and 4 files with indirect coverage changes

---

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[dboyliao]

Nice work!
Thanks @stringsbuilder.