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>