build: remove deprecated '// +build' tag

This has been replaced by `//build:...` for a long time now.

Removal of the old build tag was automated with:

    for i in $(git grep -l '^// +build' | grep -v -e '^vendor/'); do if ! grep -q '^// Code generated' "$i"; then sed -i -e '/^\/\/ +build/d' "$i"; fi; done

Kubernetes-commit: ad79e479c2314d1de91e54bc5630c52027f12e21

hack/tools.go

@@ -1,5 +1,4 @@
 //go:build tools
-// +build tools
 
 /*
 Copyright 2019 The Kubernetes Authors.

pkg/signals/signal_posix.go

@@ -1,5 +1,4 @@
 //go:build !windows
-// +build !windows
 
 /*
 Copyright 2017 The Kubernetes Authors.