[trivial] Simplify dict get call by removing unused default

The default value of the dictionary get method is None,
so remove the default value of None in the get method

Change-Id: Ic1b69047b6edd4987df3191ce84658393a1cb968

Author: liuhuajie

nova/network/neutron.py

@@ -2498,7 +2498,7 @@ def _ports_needed_per_instance(self, context, neutron, requested_networks):
                 if request.port_id:
                     port = self._show_port(context, request.port_id,
                                            neutron_client=neutron)
-                    if port.get('device_id', None):
+                    if port.get('device_id'):
                         raise exception.PortInUse(port_id=request.port_id)
 
                     deferred_ip = port.get('ip_allocation') == 'deferred'