Remove legacy dither flag

This wasn't really doing anything, now that legacy gradients were
removed.

Change-Id: Iad3f10c13a741aa0babfef0dafabc3074ff4f360
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/606901
Reviewed-by: Ben Wagner <bungeman@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
1 file changed