ObjC: Deprecate codec settings parameter in startDecode method.

This parameter is being removed from the C++ API, remove it from the
ObjC API also. It was never used for anything by the H264 decoder.

Bug: webrtc:9107
Change-Id: I5222eac932a4e7d4129d803f8126b5e8d0b027b6
Reviewed-on: https://webrtc-review.googlesource.com/66740
Reviewed-by: Kári Helgason <kthelgason@webrtc.org>
Commit-Queue: Anders Carlsson <andersc@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#22730}

sdk/objc/Framework/Classes/VideoToolbox/RTCVideoDecoderH264.mm

@@ -92,6 +92,10 @@ void decompressionOutputCallback(void *decoderRef,
   [self setVideoFormat:nullptr];
 }
 
+- (NSInteger)startDecodeWithNumberOfCores:(int)numberOfCores {
+  return WEBRTC_VIDEO_CODEC_OK;
+}
+
 - (NSInteger)startDecodeWithSettings:(RTCVideoEncoderSettings *)settings
                        numberOfCores:(int)numberOfCores {
   return WEBRTC_VIDEO_CODEC_OK;