ovn: Delete stale MAC_Bindings that result in Referential Integrity Violation

ovn: Delete stale MAC_Bindings that result in Referential Integrity Violation

The MAC_Bindings have a strong reference to the Datapath_Binding. However the
MAC_Bindings are never deleted anywhere, and when the Datapath (associated
with a MAC_Binding) is deleted, the ovsdb-server returns Referential
Integrity Violation. This prevents newer operations initiated from the CMS
from being committed to the Southbound DB.

The patch fixes this  by deleting the MAC_Binding entry when the
logical_port referred in the mac_binding entry is deleted.

Signed-off-by: Chandra Sekhar Vejendla <csvejend@us.ibm.com>
Signed-off-by: Ben Pfaff <blp@ovn.org>