Remove kIndex_8_GrPixelConfig

It's been disabled for a long time (GPUs don't support it, and it actually
caused performance regression in testing).

BUG=skia:4333

Change-Id: I6e2bf755f765168fd616de6c9c023c6fbd5abd20
Reviewed-on: https://skia-review.googlesource.com/6897
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
11 files changed