mwan3: fix disconnected event generation in mwan3track
Before this change two disconnected events were generated. This is wrong!
The disconnected event is impliciet generated by the hotplug script on ifdown
event. The mwan3track script is notified by a USR1 signal which
generates the disconnectd event. The additional "disconnectd" event on
ifdown is not required.
Signed-off-by: Florian Eckert <fe@dev.tdt.de>