Updated ScopedVkLoaderEnvironment destructor to delete an environment variable
that gets set during initialization of the renderer to load the Mock/Null
driver for tests that request it.

Bug: angleproject:2698
Change-Id: Ibbac795b6315971b303d97a55d24565a403d056c
Reviewed-on: https://chromium-review.googlesource.com/1120940Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Tobin Ehlis <tobine@google.com>
Commit-Queue: Jamie Madill <jmadill@chromium.org>