Remove unnecessary video factory references in PeerConnectionFactory

The video codec factories should be owned by the video engine instead
of by the PeerConnectionFactory.

Bug: None
Change-Id: If63d47cef565138d51377af3fc9ea973950c9390
Reviewed-on: https://webrtc-review.googlesource.com/1601
Reviewed-by: Anders Carlsson <andersc@webrtc.org>
Reviewed-by: Taylor Brandstetter <deadbeef@webrtc.org>
Commit-Queue: Magnus Jedvert <magjed@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#20002}

sdk/objc/Framework/Classes/PeerConnection/RTCPeerConnectionFactory.mm

@@ -110,8 +110,6 @@
         _networkThread.get(),
         _workerThread.get(),
         _signalingThread.get(),
-        nullptr,  // video_encoder_factory
-        nullptr,  // video_decoder_factory
         std::unique_ptr<cricket::MediaEngineInterface>(),
         std::unique_ptr<webrtc::CallFactoryInterface>(),
         std::unique_ptr<webrtc::RtcEventLogFactoryInterface>());