Get rid of mmd->audio_format.

Get rid of mmd->audio_format.

The current audio format is also stored in mmd->afd.afsi.audio_format_id,
so use the value stored there. However, most users of mmd->audio_format
used this value only to detect whether an audio file is open ATM. In
vss.c, use the map pointer instead to find out whether an audio file is
open.