Revert of SkNx miplevel building (patchset #3 id:40001 of https://codereview.chromium...

Revert of SkNx miplevel building (patchset #3 id:40001 of https://codereview.chromium.org/1606013003/ )

Reason for revert:
Paranoid revert to see if it helps skia:4823

Original issue's description:
> SkNx miplevel building
>
> All sizes approximately twice as fast.
>
> Before:
>     micros   	bench
>    1649.35  	mipmap_build_512x512	nonrendering
>    1824.42  	mipmap_build_511x512	nonrendering
>    2100.66 ?	mipmap_build_512x511	nonrendering
>    2375.94  	mipmap_build_511x511	nonrendering
>
> After:
>     micros   	bench
>     730.32 !	mipmap_build_512x512	nonrendering
>     922.12  	mipmap_build_511x512	nonrendering
>     999.07  	mipmap_build_512x511	nonrendering
>    1342.93 !	mipmap_build_511x511	nonrendering
>
>
> BUG=skia:
> GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1606013003
> CQ_EXTRA_TRYBOTS=client.skia:Test-Ubuntu-GCC-GCE-CPU-AVX2-x86_64-Release-SKNX_NO_SIMD-Trybot
>
> Committed: https://skia.googlesource.com/skia/+/3bd5aba2a0e165997f683cf3aa306661e71464f6

TBR=reed@google.com,mtklein@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/1607323002