demux: get rid of DEMUXER_CTRL_GET_TIME_LENGTH

Similar purpose as f34e1a0deea45e.

Somehow this is much more natural too, and needs less code.

This breaks runtime updates to duration. This could easily be fixed, but
no important demuxer does this anyway. Only demux_raw and demux_disc
might (the latter for BD/DVD). For the latter it might actually have
some importance when changing titles at runtime (I guess?), but guess
what, I don't care.