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>
diff --git a/src/gpu/gl/GrGLTexture.cpp b/src/gpu/gl/GrGLTexture.cpp
index a5a1a37..5473980 100644
--- a/src/gpu/gl/GrGLTexture.cpp
+++ b/src/gpu/gl/GrGLTexture.cpp
@@ -50,7 +50,7 @@
         , fParameters(sk_make_sp<GrGLTextureParameters>()) {
     this->init(desc);
     this->registerWithCache(budgeted);
-    if (GrGLFormatIsCompressed(desc.fFormat)) {
+    if (GrPixelConfigIsCompressed(desc.fConfig)) {
         this->setReadOnly();
     }
 }