refactor: remove obsolete // +build tag

[zjumathcode]

From Go 1.17, the preferred syntax for build constraints is //go:build,
which replaces the old // +build form. The old style is now considered
deprecated but still supported for backward compatibility.

This change removes the obsolete // +build xxx line, keeping only the
modern //go:build xxx directive.

More info: golang/go#41184 and https://go.dev/doc/go1.17#build-lines

Design Doc / Proposal：
https://go.dev/design/draft-gobuild

Implementation details

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