Remove support for image load/store

This isn't used and has become a maintenance burden.

Change-Id: I5f3af8f91e5c4f073fe4ea30e0a7f1f61efeea47
Reviewed-on: https://skia-review.googlesource.com/70640
Reviewed-by: Robert Phillips <robertphillips@google.com>
Commit-Queue: Brian Salomon <bsalomon@google.com>
44 files changed