QMP: Drop DEBUG event

This event has been introduced in the first round of QMP commits,
turns out that it's based on the usage of the EXCP_DEBUG macro,
which has discussable semantics when exposed through QMP.

As libvirt doesn't use this, let's just drop it.

Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>

monitor.c

@@ -399,9 +399,6 @@ void monitor_protocol_event(MonitorEvent event, QObject *data)
     assert(event < QEVENT_MAX);
 
     switch (event) {
-        case QEVENT_DEBUG:
-            event_name = "DEBUG";
-            break;
         case QEVENT_SHUTDOWN:
             event_name = "SHUTDOWN";
             break;