This class provides no added benefit now that we've scrapped the
type-specialized classes. We can squash it's functionality into
TextureStorage instead.

BUG=angle:741

Change-Id: I04984806719a01f299986265fe09b3cccf04dca5
Reviewed-on: https://chromium-review.googlesource.com/218316Tested-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>