Remove MoltenVK support

We haven't tested this in a while. Use the Metal backend instead.

Change-Id: Ic06bf4faff99950bd24e41bdea03962db99cfeee
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/277612
Commit-Queue: Brian Salomon <bsalomon@google.com>
Reviewed-by: Jim Van Verth <jvanverth@google.com>
15 files changed