Revert of Fix to set correct output type when blending when we've read dst (patchset #4 id:60001 of https://codereview.chromium.org/791143002/)

Reason for revert:
Breaks DM on windows

Original issue's description:
> Fix to set correct output type when blending when we've read dst
>
> BUG=skia:
>
> Committed: https://skia.googlesource.com/skia/+/733fc2054044377e914559fa4f46db66ab7d9cea

TBR=bsalomon@google.com,egdaniel@google.com
NOTREECHECKS=true
NOTRY=true
BUG=skia:

Review URL: https://codereview.chromium.org/795783002