Remove SELECT FOR UPDATE use in delete_firewall

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