Remove GrPendingIOResource

AFAICT these three remaining uses of GrPendingIOResource were just real-ref placeholders (i.e., they just added 1 pending read and then held on to the GrPendingIOResource until object destruction).

Change-Id: Iba6bb199477d846d079999f09794422e240c675a
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/235458
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>
16 files changed