Delete disassociated floating ips on external network deletion

Also remove redundant check for network in use in bigswitch plugin

Closes-Bug: #1238439
Closes-Bug: #1259144

Change-Id: I7586f43c2e99be9df491c68bf1e8658994ffd263

neutron/plugins/ml2/plugin.py

@@ -444,6 +444,8 @@ class Ml2Plugin(db_base_plugin_v2.NeutronDbPluginV2,
         while True:
             try:
                 with session.begin(subtransactions=True):
+                    self._process_l3_delete(context, id)
+
                     # Get ports to auto-delete.
                     ports = (session.query(models_v2.Port).
                              enable_eagerloads(False).