Remove pixel config fallback - failing is a better option.

This produces surprising results in some cases. For example: Attempt to
make an F16 SkSurface on ANGLE. We can't render to F16, but this succeeds
and gives you back an N32 surface instead. Ran all tests and GMs in DM on
ANGLE (where this is most likely to be an issue). No problems.

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2272423004

Review-Url: https://codereview.chromium.org/2272423004
1 file changed