Remove GrFragmentProcessor::addAndPushFlagToChildren, dead code

The last flag that was pushed down was removed here:
https://skia-review.googlesource.com/c/skia/+/435018

Bug: skia:12198
Change-Id: Ia184d6a03fe4078e00dd92d9d3cd87596d0c0fcd
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/436999
Commit-Queue: Brian Salomon <bsalomon@google.com>
Commit-Queue: John Stiles <johnstiles@google.com>
Auto-Submit: Brian Salomon <bsalomon@google.com>
Reviewed-by: John Stiles <johnstiles@google.com>
2 files changed