all: cleanup unused code, part 2 (#10670)

And enable U1000 check in staticcheck.

Updates #cleanup

Signed-off-by: Andrew Lytvynov <awly@tailscale.com>

util/linuxfw/helpers.go

@@ -1,6 +1,8 @@
 // Copyright (c) Tailscale Inc & AUTHORS
 // SPDX-License-Identifier: BSD-3-Clause
 
+//go:build linux
+
 package linuxfw
 
 import (

util/linuxfw/linuxfw_unsupported.go

@@ -5,7 +5,7 @@
 // support in upstream dependencies.
 
 // TODO(#8502): add support for more architectures
-//go:build !linux || (linux && !(arm64 || amd64))
+//go:build linux && !(arm64 || amd64)
 
 package linuxfw