Remove unused/barely-supported compressed GrPixelConfigs

Change-Id: Ie7b813c8b0523ae53cf9f2328645ddf4c5af49a8
Reviewed-on: https://skia-review.googlesource.com/8326
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>
8 files changed