Remove arithmetic mode GrXP code.

The arithmetic mode xfermode is only used as an implementation detail of SkXfermodeImageFilter which always uses the arithmetic FP.

Change-Id: I5f9607aa9731a21e6666d9c749dfa705d40d5775
Reviewed-on: https://skia-review.googlesource.com/6688
Commit-Queue: Brian Salomon <bsalomon@google.com>
Reviewed-by: Greg Daniel <egdaniel@google.com>
4 files changed