remove unused variants in imagefilter factories

Bug: skia:
Change-Id: Ife12bfa1c16f9dd87b24f73d3b260b0d2f46c7df
Reviewed-on: https://skia-review.googlesource.com/20207
Reviewed-by: Mike Reed <reed@google.com>
Commit-Queue: Mike Reed <reed@google.com>
4 files changed