remove (empty) SkXfermode.h

BUG=skia:

Change-Id: Ic5ceb829a80252b35d4a953dc32417db9ddc8ec8
Reviewed-on: https://skia-review.googlesource.com/5481
Reviewed-by: Mike Reed <reed@google.com>
Commit-Queue: Mike Reed <reed@google.com>
1 file changed