ObjC: Remove RTCVideoFrame.nativeHandle

Access to a native buffer should be through RTCVideoFrame.buffer
instead.

BUG=webrtc:7785

Review-Url: https://codereview.webrtc.org/2990253002
Cr-Commit-Position: refs/heads/master@{#19430}

webrtc/sdk/objc/Framework/Classes/PeerConnection/RTCVideoFrame.mm

@@ -83,14 +83,6 @@
   return _timeStampNs;
 }
 
-- (CVPixelBufferRef)nativeHandle {
-  if ([_buffer isKindOfClass:[RTCCVPixelBuffer class]]) {
-    return ((RTCCVPixelBuffer *)_buffer).pixelBuffer;
-  } else {
-    return nullptr;
-  }
-}
-
 - (RTCVideoFrame *)newI420VideoFrame {
   return [[RTCVideoFrame alloc] initWithBuffer:[_buffer toI420]
                                       rotation:_rotation