Remove useless check in _rpc_update_firewall

Firewall_db_mixin.update_firewall returns an updated firewall or raises
an error (if the firewall is not found), that's why checking if its
result is evaluated to False is not required

Change-Id: Ib4e2fa0920783728b8e1cc8597b3edf78e098b52
Merge "Remove useless check in _rpc_update_firewall"