CLOUDSTACK-6761: Fixed removing proxy arp rule on deleting static nat or PF rule on ip

 The proxy-arp add/del is done on firewall rule add/del.
 The proxy-arp rule is deleted only when there is no static nat or dest nat rule is not using the ip.

 When there is static nat or PF and firewall rule
   a. Delete firewall rule. It skips delete proxy-arp because the rule is used by static nat rule.
   b. After deleting fw rule if we disable static nat there is no way to delete proxy-arp rule.

   On VM expunge we are deleting firewall rules first then static nat rules. This caused the stale proxy-arp
   rules.

   With this fix adding/deleting proxy arp rule on static nat/PF rule add/del.