Revert "Blend functions as SkSL built ins."

This reverts commit 3794db84e141861052fd5a58e518c70e9a7350a4.

Reason for revert: Still breaking Pixel3 processor optimization test

Original change's description:
> Blend functions as SkSL built ins.
> 
> GPU-only for right now.
> 
> Use in GrGLSLBlend::AppendMode.
> 
> Remove unused GrGLSLBlend::AppendRegionOp.
> 
> Change-Id: I23affe255a1adafa55f3a07ca6291dc84e18e213
> Reviewed-on: https://skia-review.googlesource.com/c/skia/+/254678
> Reviewed-by: Ethan Nicholas <ethannicholas@google.com>
> Reviewed-by: Brian Osman <brianosman@google.com>
> Commit-Queue: Brian Salomon <bsalomon@google.com>

TBR=bsalomon@google.com,brianosman@google.com,ethannicholas@google.com

Change-Id: I0675c0fe00d6540202ca0676fd9b07f3007d0404
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/260639
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Brian Salomon <bsalomon@google.com>
8 files changed