[skif] Remove legacy Merge filter implementation

Bug: b/263133201
Change-Id: I4f478f5762bf7befb516f9d956a0fa18d6c468b7
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/703196
Auto-Submit: Michael Ludwig <michaelludwig@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Michael Ludwig <michaelludwig@google.com>
2 files changed