runtime: remove unused signals_windows.h

Change-Id: I35fe76661c80ca808a711acf608a23c77aeb0608
Reviewed-on: https://go-review.googlesource.com/4651Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/runtime/signals_windows.h

-// Copyright 2011 The Go Authors. All rights reserved.
-// Use of this source code is governed by a BSD-style
-// license that can be found in the LICENSE file.