Remove alpha versions of constant blend coeffs.

Change-Id: Iccff036e59f76f7d9651d7fd6ec005d329e3b593
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/280349
Commit-Queue: Greg Daniel <egdaniel@google.com>
Reviewed-by: Chris Dalton <csmartdalton@google.com>
7 files changed