gvfsproxyvolumemonitordaemon: Turn off debug

Now that dbus-activated session services have their stdout/stderr
connected to the systemd journal (as of recent dbus), these debug
messages now show up by default.

The #define DEBUG_ENABLED was apparently turned on by accident in
commit 8315eaf84e52c6a00a4905f1f6a73182db9e8015 - we can just turn it
off again.

(Why is this not just g_debug()?  Who knows...)

https://bugzilla.gnome.org/show_bug.cgi?id=711365