refactor: remove obsolete // +build tag

[xiaolinny]

Before asking for reviews, here is a check list of the most common things you might need to consider:

• updating the Changelog
• writing unit tests
• checking if your changes affect other coins or tokens in unintended ways
• testing on multiple environments (Qt, Android, ...)
• having an AI review your changes

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

[benma]

There are more instances of this.

[xiaolinny]

There are more instances of this.

Oh, indeed. Thank you for your reminder. 😄 @benma

I've made all the modifications. Please review again.