remove unneeded code for index8 imagse

Bug: skia:6828
Change-Id: I039d6bc35a1ed93ce747247f32fe4e9d5b09da0c
Reviewed-on: https://skia-review.googlesource.com/21400
Reviewed-by: Florin Malita <fmalita@chromium.org>
Commit-Queue: Mike Reed <reed@google.com>
14 files changed