remove unused api on xfermodeimagefilter

Bug: skia:
Change-Id: If99ee7b4d959d728849a20ee43a0d0ec25196f58
Reviewed-on: https://skia-review.googlesource.com/20303
Reviewed-by: Mike Reed <reed@google.com>
Commit-Queue: Mike Reed <reed@google.com>
1 file changed