Don't explicitly store Auditioner -> Monitor connections.

Don't explicitly store Auditioner -> Monitor connections.

The monitor section connection is implicit and done by default.
This fixes an issue with the auditioner connections being lost when
switching between sessions that use/don't use the monitor-section.

Previously:
1) load session with monitor section, save session
  -> global config explicitly saved
  "auditioner-output-left" -> "ardour:Monitor/audio_in 1"

2) Load a session w/o monitor-section. The config is parsed,
  "ardour:Monitor/audio_in" port does not exist, connection was lost.