codec: Remove deprecated usage of AVCodecContext::refcounted_frames

This was only necessary for use with the
avcodec_decode_video2/avcoded_decode_audio4 APIs which are also
deprecated.

Given we use avcodec_send_packet/avcodec_receive_frame, this isn't
necessary, this is even indicated directly within the FFmpeg API changes
document here on 2017-09-26:

https://github.com/FFmpeg/FFmpeg/blob/master/doc/APIchanges#L410

This prevents our code from breaking whenever we update to a newer
version of FFmpeg in the future if they ever decide to fully remove this
API member.