This was useless for anything but the raw demuxers. In most cases, this
would most likely lead to display of bogus duration values, because the
bitrates used are per-track, not the total file bitrate. There was
actually no case left where this code was helpful.
Note that demux_lavf has its own code for this using the total file
bitrate. Also, mplayer.c can calculate the playback percentage from
current file position / current file size. This is not removed.
