Remove rx::TextureStorageInterface entirely. 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/218316 Tested-by: Jamie Madill <jmadill@chromium.org> Reviewed-by: Geoff Lang <geofflang@chromium.org>