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: 154a7bb877

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-Commit-Position: refs/heads/master@{#17119}

webrtc/sdk/objc/Framework/Classes/Metal/RTCMTLVideoView.m

@@ -51,9 +51,9 @@
 #pragma mark - Private
 
 + (BOOL)isMetalAvailable {
-#if defined(RTC_SUPPORTS_METAL)
+#if defined(__OBJC__) && COREVIDEO_SUPPORTS_METAL
   return YES;
-#else
+#elif
   return NO;
 #endif
 }