Revert "Use RasterPipeline to support full precision on 16-bit RGBA pngs"

Revert "Use RasterPipeline to support full precision on 16-bit RGBA pngs"

This reverts commit bb2339da39ab3ee59121acd911920dafcd4a2f72.

Reason for revert: Breaks MSAN

Original change's description:
> Use RasterPipeline to support full precision on 16-bit RGBA pngs
>
> TODO: Support more precision on 16-bit RGB pngs
>
> BUG=skia:
>
> CQ_INCLUDE_TRYBOTS=skia.primary:Test-Ubuntu-GCC-GCE-CPU-AVX2-x86_64-Release-SKNX_NO_SIMD
>
> Change-Id: I89dfef3b4887b9c4895c17309933883ab90ffa4d
> Reviewed-on: https://skia-review.googlesource.com/6260
> Reviewed-by: Mike Reed <reed@google.com>
> Reviewed-by: Leon Scroggins <scroggo@google.com>
> Reviewed-by: Mike Klein <mtklein@chromium.org>
> Commit-Queue: Matt Sarett <msarett@google.com>
>

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

Change-Id: I47579c20af033a75883e2b35567cb9c690ce54b0
Reviewed-on: https://skia-review.googlesource.com/6975
Commit-Queue: Matt Sarett <msarett@google.com>
Reviewed-by: Matt Sarett <msarett@google.com>