Fix bug in GrClearOp combining and remove some asserts

The buffer combining code path was combining the ops but never
telling the external system that the second op could be removed.

Bug: skia:10963
Change-Id: If015d877ffbbb75964aae9ca92ea760d7041372a
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/339203
Commit-Queue: Robert Phillips <robertphillips@google.com>
Reviewed-by: Michael Ludwig <michaelludwig@google.com>
3 files changed