[graphite] Remove vanilla asyncReadPixels interface from Context.

Just to reduce interface cruft.

Bug: b/290198076
Change-Id: I376bbe59e8a9096cf078fa97bb51b67a76415bc1
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/720336
Commit-Queue: Jim Van Verth <jvanverth@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
3 files changed