Cleanup connection after switch deletion

[Alopalao]

After a switch is deleted, there is a leftover data in controller.connections (switch 00:00:00:00:00:00:00:01 was deleted):

kytos $> controller.connections
Out[3]: 
{('127.0.0.1',
  33734): Connection('127.0.0.1', 33734, None, Switch('00:00:00:00:00:00:00:01'), <ConnectionState.FINISHED: 4>),
 ('127.0.0.1',
  33744): Connection('127.0.0.1', 33744, <asyncio.TransportSocket fd=116, family=AddressFamily.AF_INET, type=SocketKind.SOCK_STREAM, proto=6, laddr=('127.0.0.1', 6653), raddr=('127.0.0.1', 33744)>, Switch('00:00:00:00:00:00:00:03'), <ConnectionState.ESTABLISHED: 2>),
 ('127.0.0.1',
  33750): Connection('127.0.0.1', 33750, <asyncio.TransportSocket fd=117, family=AddressFamily.AF_INET, type=SocketKind.SOCK_STREAM, proto=6, laddr=('127.0.0.1', 6653), raddr=('127.0.0.1', 33750)>, Switch('00:00:00:00:00:00:00:02'), <ConnectionState.ESTABLISHED: 2>)}

It is not a problem to keep this data, it is just a leftover.
From PR comment.

[viniarck]

Good one. Future note and requirement to be aware:

The main gotcha to be aware though is that it's not desirable to close the socket if a deleted switch conn is still ESTABLISHED, otherwise it'd close the connection with that switch, which in turn, if the network operator hand't remove the controller tcp connection there yet, it would reconnect again, leading to another (undesired) rediscovery.

Ideally, this needs to be clean up but only for non existing switches when they disconnect.