demux: fix "-demuxer mpegps", don't force demuxer in stream_dvd

Internally, stream_dvd.c returned DEMUXER_TYPE_MPEG_PS, and the same
value was hardcoded to enforced usage of demux_lavf in demux.c. But
"-demuxer mpegps" basically did the same, so that switch was broken
for this format. Undo this and don't request a demuxer in stream_dvd.c.
demux_lavf.c is (probably) good enough to probe correctly with DVD.
Otherwise, we'd actually have to do something completely different to
force the libavformat demuxer.