windows: move Unsetenv into env_windows.go

Unsetenv was in its own file for Go 1.4 compatibility. This version is
no longer supported, so move Unsetenv with the other environment
specific functions.

Change-Id: I63d67e2a9e4d7651be79c2e72a7f27fe0fac68c4
Reviewed-on: https://go-review.googlesource.com/86539
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

windows/env_windows.go

@@ -23,3 +23,7 @@ func Clearenv() {
 func Environ() []string {
 	return syscall.Environ()
 }
+
+func Unsetenv(key string) error {
+	return syscall.Unsetenv(key)
+}