runtime/cgo: do not link threads lib by default on windows

I do not know why it is included. All tests pass without it.

Change-Id: I839076ee131816dfd177570a902c69fe8fba5022
Reviewed-on: https://go-review.googlesource.com/32144
Run-TryBot: Minux Ma <minux@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/runtime/cgo/cgo.go

@@ -20,7 +20,6 @@ package cgo
 #cgo !android,linux LDFLAGS: -lpthread
 #cgo netbsd LDFLAGS: -lpthread
 #cgo openbsd LDFLAGS: -lpthread
-#cgo windows LDFLAGS: -mthreads
 
 #cgo CFLAGS: -Wall -Werror