Revert of Add Metal video view in AppRTCMobile and metal availability macro. (patchset #5 id:80001 of https://codereview.webrtc.org/2722583002/ )

Reason for revert:
Breaks AppRTCMobile

Original issue's description:
> Add Metal video view in AppRTCMobile and Metal availability macro.
>
> - The RTC_SUPPORTS_METAL macro allows consumers to gracefully handle compilation for different archs that are not supporting Metal.
>
> BUG=webrtc:7079
>
> Review-Url: https://codereview.webrtc.org/2722583002
> Cr-Commit-Position: refs/heads/master@{#17004}
> Committed: https://chromium.googlesource.com/external/webrtc/+/154a7bb877abefada22afe28b0a755cf621e8d9e

TBR=magjed@webrtc.org,tkchin@webrtc.org,denicija@webrtc.org
# Not skipping CQ checks because original CL landed more than 1 days ago.
BUG=webrtc:7079

Review-Url: https://codereview.webrtc.org/2739793003
Cr-Original-Commit-Position: refs/heads/master@{#17119}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: cbbd8c76e8a8a1cf3f1193ea50980bc9d858458d
4 files changed