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>