A controller should delete its interfaces if necessary

This was causing controlnet.py to not clean up its interfaces, since
those interfaces were linked from the controller to the switch
rather than vice-versa!!

deleteIntfs already will only delete interfaces whose names match
our name, which should usually prevent us from accidentally deleting
a hardware interface from the root namespace!

mininet/node.py

@@ -1425,7 +1425,6 @@ class Controller( Node ):
         "Stop controller."
         self.cmd( 'kill %' + self.command )
         self.cmd( 'wait %' + self.command )
-        kwargs.update( deleteIntfs=False )
         super( Controller, self ).stop( *args, **kwargs )
 
     def IP( self, intf=None ):