Revert "Improve thread-safety of MTL Renderer."

This reverts commit a8f13ccad400eb8ff84a379042c0595951ca9658.

Reason for revert: It's causing no video to be shown after the 1st call.

Original change's description:
> Improve thread-safety of MTL Renderer.
> 
> Bug: b/77579859
> Change-Id: I427d0f41593155dc5cbf98a09d7ec826497b803c
> Reviewed-on: https://webrtc-review.googlesource.com/67040
> Commit-Queue: Kári Helgason <kthelgason@webrtc.org>
> Reviewed-by: Anders Carlsson <andersc@webrtc.org>
> Cr-Commit-Position: refs/heads/master@{#22795}

TBR=andersc@webrtc.org,kthelgason@webrtc.org

Change-Id: Ia8f33995e087178f1c3be7753f70be8ba18447f8
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: b/77579859
Reviewed-on: https://webrtc-review.googlesource.com/68860
Reviewed-by: JT Teh <jtteh@webrtc.org>
Commit-Queue: JT Teh <jtteh@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#22800}

sdk/objc/Framework/Classes/Metal/RTCMTLNV12Renderer.mm

@@ -18,7 +18,6 @@
 #import "WebRTC/RTCVideoFrameBuffer.h"
 
 #import "RTCMTLRenderer+Private.h"
-#include "rtc_base/checks.h"
 
 #define MTL_STRINGIFY(s) @ #s
 
@@ -86,7 +85,6 @@ static NSString *const shaderSource = MTL_STRINGIFY(
 }
 
 - (BOOL)setupTexturesForFrame:(nonnull RTCVideoFrame *)frame {
-  RTC_DCHECK([frame.buffer isKindOfClass:[RTCCVPixelBuffer class]]);
   [super setupTexturesForFrame:frame];
   CVPixelBufferRef pixelBuffer = ((RTCCVPixelBuffer *)frame.buffer).pixelBuffer;