Revert "SkRasterPipelineBlitter: support A8"

Revert "SkRasterPipelineBlitter: support A8"

This reverts commit f44373c119290b501d4aec7385e16d12c28a1f0f.

Reason for revert: MSAN

Original change's description:
> SkRasterPipelineBlitter: support A8
>
> This adds support for loading and storing A8, then uses it in SkRasterPipelineBlitter.
>
> I think this handles all dst formats now: A8, 565, 8888 (by policy, sRGB only) and F16.
>
> CQ_INCLUDE_TRYBOTS=master.tryserver.blink:linux_trusty_blink_rel;skia.primary:Test-Ubuntu-GCC-GCE-CPU-AVX2-x86_64-Release-SKNX_NO_SIMD
>
>
> Change-Id: Id207f6e6c56b6bfcc301d77dd23e0959bb7afba8
> Reviewed-on: https://skia-review.googlesource.com/6554
> Reviewed-by: Mike Reed <reed@google.com>
> Commit-Queue: Mike Klein <mtklein@chromium.org>
>

TBR=mtklein@chromium.org,reed@google.com,reviews@skia.org
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true

Change-Id: I9ead3c3335e1776e9a1639ca0481253821505d67
Reviewed-on: https://skia-review.googlesource.com/6625
Commit-Queue: Mike Klein <mtklein@chromium.org>
Reviewed-by: Mike Klein <mtklein@chromium.org>