audio: remove LOG_TO_MONITOR along with default_mon

Setting QEMU_AUDIO_LOG_TO_MONITOR=1 can crash qemu (if qemu tries to log
to the monitor before it's being initialized), and also nothing else in
qemu logs to the monitor.

This log to monitor feature was the last thing that used the default_mon
variable, so I removed it too (as using it can cause problems).

Signed-off-by: Kővágó, Zoltán <DirtY.iCE.hu@gmail.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>