Remove unused GrFlushFlags.

This also adds back default flush() calls which simply do a flush
without any submit.

Change-Id: Ia8c92bbdecd515d871abfa6364592f502e98656b
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/298818
Commit-Queue: Greg Daniel <egdaniel@google.com>
Reviewed-by: Brian Salomon <bsalomon@google.com>
Reviewed-by: John Stiles <johnstiles@google.com>
19 files changed