all: remove old +build tags

The //go:build syntax was introduced in Go 1.17:

https://go.dev/doc/go1.17#build-lines

gofmt has kept the +build and go:build lines in sync since
then, but enough time has passed. Time to remove them.

Done with:

    perl -i -npe 's,^// \+build.*\n,,' $(git grep -l -F '+build')

Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

util/cstruct/cstruct_example_test.go

@@ -5,7 +5,6 @@
 // Only built on 64-bit platforms to avoid complexity
 
 //go:build amd64 || arm64 || mips64le || ppc64le || riscv64
-// +build amd64 arm64 mips64le ppc64le riscv64
 
 package cstruct
 

util/endian/big.go

@@ -3,7 +3,6 @@
 // license that can be found in the LICENSE file.
 
 //go:build mips || mips64 || ppc64 || s390x
-// +build mips mips64 ppc64 s390x
 
 package endian
 

util/endian/little.go

@@ -3,7 +3,6 @@
 // license that can be found in the LICENSE file.
 
 //go:build 386 || amd64 || arm || arm64 || mips64le || mipsle || ppc64le || riscv64 || wasm
-// +build 386 amd64 arm arm64 mips64le mipsle ppc64le riscv64 wasm
 
 package endian
 

util/osshare/filesharingstatus_noop.go

@@ -3,7 +3,6 @@
 // license that can be found in the LICENSE file.
 
 //go:build !windows
-// +build !windows
 
 package osshare
 

util/osshare/filesharingstatus_windows.go

@@ -3,7 +3,6 @@
 // license that can be found in the LICENSE file.
 
 //go:build windows
-// +build windows
 
 package osshare
 

util/pidowner/pidowner_noimpl.go

@@ -3,7 +3,6 @@
 // license that can be found in the LICENSE file.
 
 //go:build !windows && !linux
-// +build !windows,!linux
 
 package pidowner
 

util/racebuild/off.go

@@ -3,7 +3,6 @@
 // license that can be found in the LICENSE file.
 
 //go:build !race
-// +build !race
 
 package racebuild
 

util/racebuild/on.go

@@ -3,7 +3,6 @@
 // license that can be found in the LICENSE file.
 
 //go:build race
-// +build race
 
 package racebuild
 

util/systemd/systemd_linux.go

@@ -3,7 +3,6 @@
 // license that can be found in the LICENSE file.
 
 //go:build linux
-// +build linux
 
 package systemd
 

util/systemd/systemd_nonlinux.go

@@ -3,7 +3,6 @@
 // license that can be found in the LICENSE file.
 
 //go:build !linux
-// +build !linux
 
 package systemd
 

util/winutil/winutil_notwindows.go

@@ -3,7 +3,6 @@
 // license that can be found in the LICENSE file.
 
 //go:build !windows
-// +build !windows
 
 package winutil