Remove unused copy-to-external YUV SkImage factories.

Also inline some helper functions into newer factory method
that are no longer shared.

Bug: skia:10632
Change-Id: I466c59f668d882802087acad1ec1f229505a3377
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/334596
Reviewed-by: Jim Van Verth <jvanverth@google.com>
Commit-Queue: Brian Salomon <bsalomon@google.com>
6 files changed