Revert "Add ETC2 support to Metal backend."

This reverts commit c25802db30fa371d3984eeef6b04a605e7b7f51f.

Reason for revert: Causing assert in MtlBackendAllocationTest.

Original change's description:
> Add ETC2 support to Metal backend.
> 
> Fills out onCreateCompressedTexture and sets iOS caps to support ETC2.
> Skia supports no compressed texture formats on MacOS as yet.
> 
> Bug: skia:8243
> Change-Id: I2ce20f601c035a8822e658c88b815fdd8587aa98
> Reviewed-on: https://skia-review.googlesource.com/c/skia/+/240692
> Commit-Queue: Jim Van Verth <jvanverth@google.com>
> Reviewed-by: Brian Salomon <bsalomon@google.com>

TBR=jvanverth@google.com,bsalomon@google.com

Change-Id: I45adf78a7106be69bd8a6437f61abe28eddedbb2
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: skia:8243
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/241877
Reviewed-by: Jim Van Verth <jvanverth@google.com>
Commit-Queue: Jim Van Verth <jvanverth@google.com>
8 files changed