Cleanup GrPixelConfig helper functions

Remove unused functions, and convert another to switch style.

BUG=skia:

Change-Id: I93edbece54f147d868801bb008e6f935a8cb997f
Reviewed-on: https://skia-review.googlesource.com/6947
Reviewed-by: Brian Salomon <bsalomon@google.com>
Reviewed-by: Robert Phillips <robertphillips@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
2 files changed