[graphite] Remove redundant Image constructor to match Image_YUVA.

Change-Id: I3f504659ff36e86543a98be07148d8dcb77cf65b
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/706538
Commit-Queue: Jim Van Verth <jvanverth@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
5 files changed