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>