In the past firewall_driver setting was configured for ML2 plugin
because it was used in the
neutron.agent.securitygroups_rpc.is_firewall_enabled() function but
currently it's not needed anymore as there is other config option
"enable_security_group" for that.
Related-bug: #1996748
Change-Id: I9b09c6afb3f1f1c33d1bdfea52ba6f4c0d0cf2dc