Remove deprecated Metal GrBackendRenderTarget constructor

Change-Id: Ib1183e1f29bc6f9ba62258e29c2a881f7bc0c89b
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/741879
Reviewed-by: Jim Van Verth <jvanverth@google.com>
Commit-Queue: Jim Van Verth <jvanverth@google.com>
Auto-Submit: Brian Osman <brianosman@google.com>
3 files changed