Remove Ganesh code from SkBlendMode/XferMode

SkBlendMode_AsXPFactory -> GrXferProcessor::FromBlendMode

This removes some unused code leftover from an XferMode -> BlendMode
migration.

Change-Id: I0405b26ed493eebbc335742fdf018ae4b609fab7
Bug: skia:14317
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/709204
Reviewed-by: Michael Ludwig <michaelludwig@google.com>
Auto-Submit: Kevin Lubick <kjlubick@google.com>
Commit-Queue: Michael Ludwig <michaelludwig@google.com>