[skif] Delete SkImageFilters::AlphaThreshold

Bug: b/263139164
Change-Id: If266007a4e76869bdfd85e5d3d20196f011884ef
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/709918
Reviewed-by: Robert Phillips <robertphillips@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Michael Ludwig <michaelludwig@google.com>
14 files changed