Revert of Enable sRGB on iOS, make sRGB decode support optional (patchset #12 id...

Revert of Enable sRGB on iOS, make sRGB decode support optional (patchset #12 id:220001 of https://codereview.chromium.org/2539993002/ )

Reason for revert:
Command Buffer, too...

Original issue's description:
> Two (related) changes here:
>
> 1) Our older iOS devices failed our sRGB tests, due to precision issues
> with alpha. At this point, we only test on iPadMini 4, and that appears
> not to have any problems.
>
> 2) iOS devices still don't have the sRGB texture decode extension. But,
> some clients have no interest in mixing legacy/color-correct rendering,
> and would like to use sRGB on these devices. This GrContextOptions flag
> enables sRGB support in those cases.
>
> Adjust the test code to produce sRGB capable contexts on these devices,
> but only for configs that have a color space. (See comment).
>
> BUG=skia:4148
>
> Committed: https://skia.googlesource.com/skia/+/9db12d2341f3f8722c8b90b11dd4cce138a8a64e
> Committed: https://skia.googlesource.com/skia/+/1aeb78c5d978b35b256525b711edd942bce01444

TBR=bsalomon@google.com
# Skipping CQ checks because original CL landed less than 1 days ago.
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=skia:4148

Review-Url: https://codereview.chromium.org/2546783005