Remove SkXfermode and SkXfermode_opts

Change-Id: Ibfa1f5af4a2cbe082d36d2a547fb6b1b86de147d
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/676137
Reviewed-by: Kevin Lubick <kjlubick@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
15 files changed