Remove support for decoding to kIndex_8

Fix up callsites, and remove tests that no longer make sense.

Bug: skia:6828
Change-Id: I2548c4b7528b7b1be7412563156f27b52c9d4295
Reviewed-on: https://skia-review.googlesource.com/21664
Reviewed-by: Derek Sollenberger <djsollen@google.com>
Commit-Queue: Leon Scroggins <scroggo@google.com>
47 files changed