*: remove old-style build directives

The newer `go:build` directives have been supported since go 1.17, and
the old ones have been deprecated in go 1.18. This commit removes all
old-style directives.

Epic: none
Release note: None

build/GNUmakefile.obsolete

@@ -558,7 +558,6 @@ $(BASE_CGO_FLAGS_FILES): Makefile build/defs.mk.sig | bin/.submodules-initialize
 	@echo '// GENERATED FILE DO NOT EDIT' > $@
 	@echo >> $@
 	@echo '//go:build $(if $(findstring $(native-tag),$@),$(native-tag),!make)' >> $@
-	@echo '// +build $(if $(findstring $(native-tag),$@),$(native-tag),!make)' >> $@
 	@echo >> $@
 	@echo 'package $(if $($(@D)-package),$($(@D)-package),$(notdir $(@D)))' >> $@
 	@echo >> $@