remove unused xfermode methods

CQ_INCLUDE_TRYBOTS=skia.primary:Test-Ubuntu-GCC-GCE-CPU-AVX2-x86_64-Release-SKNX_NO_SIMD
Bug: skia:
Change-Id: Ibc7d581bcc40134ee7cf57bb65fee2d70e119bc7
Reviewed-on: https://skia-review.googlesource.com/18842
Reviewed-by: Mike Reed <reed@google.com>
Commit-Queue: Mike Reed <reed@google.com>
4 files changed