This reverts commit 2ec3e0a1.

Reason for revert: Test suite ImageTestMetal never instantiated https://anglebug.com/5814

Original change's description:
> Metal: Support importing external metal textures
>
> via KHR_image_base and ANGLE_metal_texture_client_buffer
>
> Bug: angleproject:5763
> Change-Id: I4d4a88cfbb77d8b7508b787c7fec44073d3b11b0
> Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2757811
> Commit-Queue: Le Hoang Quyen <le.hoang.q@gmail.com>
> Reviewed-by: Kenneth Russell <kbr@chromium.org>
> Reviewed-by: Jamie Madill <jmadill@chromium.org>

Bug: angleproject:5763, angleproject:5814
Change-Id: Ia862aa37a6c914f6e21f9b3121c7c36d449475ea
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2801155
Bot-Commit: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
Reviewed-by: Kenneth Russell <kbr@chromium.org>
Commit-Queue: Kenneth Russell <kbr@chromium.org>