remove unused "+build" tags

[stklcode]

We require Go >= 1.18 since v1.0.0 and do have redundant "go:build" tags present, so no need to provide legacy build tags.

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Reference:

Since the release of Go 1.18 marks the end of support for Go 1.16, all supported versions of Go now understand //go:build lines. In Go 1.18, go fix now removes the now-obsolete // +build lines in modules declaring go 1.18 or later in their go.mod files.

https://tip.golang.org/doc/go1.18#go-build-lines