Remove unused sRGB mip-map support

Change-Id: I94b27561bfaabe821af280ddc719840e5e25d106
Reviewed-on: https://skia-review.googlesource.com/148669
Commit-Queue: Brian Osman <brianosman@google.com>
Commit-Queue: Mike Klein <mtklein@google.com>
Auto-Submit: Brian Osman <brianosman@google.com>
Reviewed-by: Mike Klein <mtklein@google.com>
4 files changed