SkXfermode no longer needs to e a flattenable

SkArithmeticMode is unreachable, so delete it

Bug: skia:
Change-Id: Ibe4c48fcf756280569c4b302da8d96bd4e21b717
Reviewed-on: https://skia-review.googlesource.com/19394
Reviewed-by: Mike Reed <reed@google.com>
Commit-Queue: Mike Reed <reed@google.com>
13 files changed