Revert "Add support for half-precision types in Metal."

This reverts commit d90e09b1ae0939b463f59f81c2958b8c08f5027b.

Reason for revert: MacMini failing CompressedBackendAllocationTest

Original change's description:
> Add support for half-precision types in Metal.
>
> This will hopefully improve performance on lower-end GPUs.
>
> Change-Id: I9c2ee6dc31acd08bec0bfb5f59edc3cf90163f9e
> Bug: skia:12339
> Reviewed-on: https://skia-review.googlesource.com/c/skia/+/465078
> Auto-Submit: John Stiles <johnstiles@google.com>
> Reviewed-by: Brian Osman <brianosman@google.com>
> Commit-Queue: John Stiles <johnstiles@google.com>

Bug: skia:12339
Change-Id: Ic5aa4bef454ca67f5ce26c600444d9565e0158cb
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/465796
Auto-Submit: Brian Osman <brianosman@google.com>
Commit-Queue: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
Bot-Commit: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
253 files changed