Revert of Make A8 readback work in more cases and improve testing. (patchset #5 id:70001 of https://codereview.chromium.org/1584563002/ )

Reason for revert:
Breaking video tests in Chrome.

Original issue's description:
> Make A8 readback work in more cases and improve testing.
>
> BUG=skia:
> GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1584563002
>
> Committed: https://skia.googlesource.com/skia/+/b76afedf11c7fe933954d030048c3222860249e1

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

Review URL: https://codereview.chromium.org/1581203005
3 files changed