Remove dead code _arp_spoofing_rule()

This code should have been removed when the allowed_address_pair
extension was added here (0efce6195fa7be80e110bd841dc9b3537a94c376).
The arp spoofing rules are handled in the method _setup_spoof_filter_chain().

Reported by: Amir Sadoughi that this was crud I left behind :)

Change-Id: Ib0e2e2a5c13fb8fa7af1f988510143f40ac335e2
Closes-bug: #1258629
Merge "Remove dead code _arp_spoofing_rule()"