Remove Floating IP DNS record upon associated port deletion

Port deletion triggers disassociate_floatingips. This patch ensures
that method not only clears the port association for a Floating IP,
but also removes any DNS record associated with it.

Change-Id: Ia6202610c09811f240af35e2523126447bf02ca5
Closes-Bug: #1812168
(cherry picked from commit 4379310846)

neutron/db/l3_db.py

@@ -1611,6 +1611,9 @@ class L3_NAT_dbonly_mixin(l3.RouterPluginBase,
                 'router_ids': router_ids,
                 'association_event': False,
             }
+            # Process DNS record removal after committing the transaction
+            if self._is_dns_integration_supported:
+                self._process_dns_floatingip_delete(context, fip.to_dict())
             registry.notify(resources.FLOATING_IP, events.AFTER_UPDATE, self,
                             **assoc_result)
         for fip in old_fips.values():

releasenotes/notes/bug-1812168-4866066106c566f4.yaml

@@ -0,0 +1,6 @@
+---
+fixes:
+  - |
+    [`bug 1812168 <https://bugs.launchpad.net/neutron/+bug/1812168>`_]
+    Remove Floating IP DNS record upon associated port deletion.
+