plan9: move Unsetenv into env_plan9.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: I8b6621993f2ab0fbafe900b19dcaee7b5b3ed26f
Reviewed-on: https://go-review.googlesource.com/86540
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

plan9/env_plan9.go

@@ -25,3 +25,7 @@ func Clearenv() {
 func Environ() []string {
 	return syscall.Environ()
 }
+
+func Unsetenv(key string) error {
+	return syscall.Unsetenv(key)
+}

plan9/env_unset.go

-// Copyright 2014 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.
-
-// +build go1.4
-
-package plan9
-
-import "syscall"
-
-func Unsetenv(key string) error {
-	// This was added in Go 1.4.
-	return syscall.Unsetenv(key)
-}