Remove SELECT FOR UPDATE use in delete_firewall

SELECT FOR UPDATE expression, which is triggered with the use of the
SQLAlchemy Query object's with_lockmode('update') method, is
detrimental to performance and scalability of the database
performance code in Neutron due to the lock contention it produces.

SELECT FOR UPDATE can be entirely avoided in delete_firewall method
with the use of single-shot DELETE expressions.

Partial-Bug: #1364358
Change-Id: Ia8db73312f5dff6a5bd573694a60a798279834cb

neutron/db/firewall/firewall_db.py

@@ -264,12 +264,11 @@ def update_firewall(self, context, id, firewall):
     def delete_firewall(self, context, id):
         LOG.debug(_("delete_firewall() called"))
         with context.session.begin(subtransactions=True):
-            fw_query = context.session.query(
-                Firewall).with_lockmode('update')
-            firewall_db = fw_query.filter_by(id=id).one()
             # Note: Plugin should ensure that it's okay to delete if the
             # firewall is active
-            context.session.delete(firewall_db)
+            count = context.session.query(Firewall).filter_by(id=id).delete()
+            if not count:
+                raise firewall.FirewallNotFound(firewall_id=id)
 
     def get_firewall(self, context, id, fields=None):
         LOG.debug(_("get_firewall() called"))