Revert of SoftLight with SkPMFloat (patchset #6 id:100001 of https://codereview.chrom...

Revert of SoftLight with SkPMFloat (patchset #6 id:100001 of https://codereview.chromium.org/1221493002/)

Reason for revert:
xfermodes and xfermodes2 show major diffs on Nexus 5 and Daisy (both ARMv7 w/NEON).  Nexus 9 and SSE all look fine...

Original issue's description:
> SoftLight with SkPMFloat
>
> SSE speeds up about 4.5x over existing integer SSE,
> NEON speeds up about 3x over serial integer code.
>
> We expect 1-2 bit component diffs in the usual GMs.
>
> Still guarded by SK_SUPPORT_LEGACY_XFERMODES,
> which I'll now try to lift in Chrome.
>
>
> BUG=skia:
>
> Committed: https://skia.googlesource.com/skia/+/3e47d49b46b3ab62071218ef3dd44642c9713e04

TBR=reed@google.com,mtklein@chromium.org
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=skia:

Review URL: https://codereview.chromium.org/1221683002