This reverts commit af66fa8fa5.
The reverted commit caused AVCodecContext.channel_layout to be set,
while requesting stereo downmix will make libavcodec output a stupid
message:
ac3: Channel layout '5.1' with 6 channels does not match specified number of channels 2: ignoring specified channel layout
The same happens with --demuxer=lavf (without this change too).
I'm not quite sure what acrobatics are required to shut up libavcodec,
but for now revert the commit. It was a rather minor, almost cosmetic
issue, which I consider less important than clean CLI terminal output.
a19307d598