Revert "dither copies when decreasing precision"

Revert "dither copies when decreasing precision"

This reverts commit 766f9cd5553ab091b85d287a79f47cbb91ad7989.

Reason for revert: unit test failures, I think on bots running portable code path

Original change's description:
> dither copies when decreasing precision
>
> Still seeing the same 4444 diffs on copyTo4444 and all_bitmap_configs,
> and now also 565 in all_bitmap_configs.
>
> BUG=chromium:720105
>
> Change-Id: I19406f57aa6d2b2f98d98c093da302b004c7cd8b
> Reviewed-on: https://skia-review.googlesource.com/17419
> Commit-Queue: Mike Klein <mtklein@chromium.org>
> Reviewed-by: Matt Sarett <msarett@google.com>
>

TBR=mtklein@chromium.org,msarett@google.com
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=chromium:720105

Change-Id: Ia9ece7dccef325233b870102ab38fbed2336b95d
Reviewed-on: https://skia-review.googlesource.com/17442
Reviewed-by: Mike Klein <mtklein@google.com>
Commit-Queue: Mike Klein <mtklein@google.com>