Revert of Re-enable CPU mipmap generation for Ganesh. Aniso mips were landed a while ago. However, the CPU bu… (patchset #1 id:1 of https://codereview.chromium.org/1835003002/ )

Reason for revert:
Speculative - seems likely to be the results of nanobench crashes on several bots.

Original issue's description:
> Re-enable CPU mipmap generation for Ganesh. Aniso mips were landed a while ago. However, the CPU builder fails when it sees Index8 (among other things), so change the code to fallback to GPU in that case. Additionally, if we're going to be mipping an sRGB image, don't use the CPU code (which is not yet gamma correct). Unfortunately, this means that we will often be using the GPU path, still - with recent codec changes, most images are coming in tagged as sRGB.
>
> BUG=skia:
> GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1835003002
>
> Committed: https://skia.googlesource.com/skia/+/b0ac1af7fab467aacbc27d20d14a09bcb960472f

TBR=reed@google.com,bsalomon@google.com,cblume@chromium.org
# 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/1836563002
3 files changed