Simplify GrProcessorSet by removing multiple color/coverage FP support.

GrProcessorSet is meant to encapsulate the processors in a GrPaint, and
GrPaint no longer supports more than one color or coverage fragment
processor on a paint.

Change-Id: I4334c967e0329503672db3b5510fce1265f25762
Bug: skia:10217
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/304857
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: John Stiles <johnstiles@google.com>
5 files changed