remove SkMixer, since we have explicit lerps/blends in SkShader

Change-Id: I937861df9d8ae89c0587ea59dd740989bebd35a4
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/207305
Reviewed-by: Florin Malita <fmalita@chromium.org>
Reviewed-by: Mike Klein <mtklein@google.com>
Commit-Queue: Mike Reed <reed@google.com>
20 files changed