remove SkXfermode from api dox -- it is gone

NOTRY=true
DOCS_PREVIEW= https://skia.org/?cl=16868
Bug: skia:1312
Change-Id: I28aafb80bcad0f3a810ca0b08caa15b70707b4a1
Reviewed-on: https://skia-review.googlesource.com/16868
Reviewed-by: Heather Miller <hcm@google.com>
Commit-Queue: Mike Reed <reed@google.com>
1 file changed