FWaaS plugin doesn't need to handle firewall rule del ops

If firewall rule is attached to firewall policy, it would raise
FirewallRuleInUse excpetion in DB ops, else it is a pure DB delete ops.
So it is useless to handle delete_firewall_rule ops in fwaas plugin.
Closes-Bug: #1322076

Change-Id: I55a000d206f232c79b41230f526007f684db8f4f
Merge "FWaaS plugin doesn't need to handle firewall rule del ops"