Remove old sRGB config support from backend render target tool utilities

Bug: skia:
Change-Id: Ie88c16ddb2b072d8bdd7efc3acabc1d9ad89012f
Reviewed-on: https://skia-review.googlesource.com/146526
Reviewed-by: Mike Klein <mtklein@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
12 files changed