Remove blank line

I'm not sure whether we really need to turn off ip_forward before
flushing iptables, but it might possibly be useful to avoid certain
unexpected behavior.

mininet/nodelib.py

@@ -94,7 +94,6 @@ class NAT( Node ):
 
         if self.flush:
             self.cmd( 'sysctl net.ipv4.ip_forward=0' )
-            
             self.cmd( 'iptables -F' )
             self.cmd( 'iptables -t nat -F' )
             # Create default entries for unmatched traffic