oops, remove the rest of SkMixer

Change-Id: I64820668e9f501002e64942d5c8a2b07abedf74a
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/207313
Reviewed-by: Mike Reed <reed@google.com>
Commit-Queue: Mike Reed <reed@google.com>
Auto-Submit: Mike Reed <reed@google.com>
2 files changed