Revert "Optimize mipmap downsample_2_2 in sRGB mode"

This reverts commit 3ea01f72f20d4f58bf0dcd420fa5c2724b67ac8d.

Reason for revert: triggering errors on the MSAN bots

Original change's description:
> Optimize mipmap downsample_2_2 in sRGB mode
> 
> Desktop (HP z620)
> Before:
> mipmap_build_2048x2048_0_gamma 10.5 ms
> mipmap_build_2048x2048_1_gamma 77.1 ms
> After:
> mipmap_build_2048x2048_0_gamma 10.5 ms
> mipmap_build_2048x2048_1_gamma 25.1 ms
> 
> Pixel XL
> Before:
> mipmap_build_2048x2048_0_gamma 160 ms
> mipmap_build_2048x2048_1_gamma 1.5 s
> After:
> mipmap_build_2048x2048_0_gamma 160 ms
> mipmap_build_2048x2048_1_gamma 313 ms
> 
> Also provides marginal performance improvements
> for other sRGB downsamples.
> 
> BUG=skia:
> 
> Change-Id: Icfcd2ccd69676ccf3822db8042a4698e4464bb71
> Reviewed-on: https://skia-review.googlesource.com/9386
> Commit-Queue: Matt Sarett <msarett@google.com>
> Reviewed-by: Mike Klein <mtklein@chromium.org>
> 

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

Change-Id: I06907fb1691077a03daadc0980e86393bc08d9c5
Reviewed-on: https://skia-review.googlesource.com/9450
Reviewed-by: Derek Sollenberger <djsollen@google.com>
Commit-Queue: Derek Sollenberger <djsollen@google.com>
4 files changed