Fix: Del interface file when interface is removed

I created an EC2 instance with only one interface, and then automatically attached a secondary interface using ubuntu-ec2net. However, if I remove the interface, and then reboot the EC2 instance, I cannot connect any more to my EC2 instance.

It was due to the eth1.cfg file referencing an interface that was not there anymore. I added a line to automatically delete that file when the interface is removed while the instance is up.