conntrack: T3579: bugfix when deleting non existent iptable rules
conntrack: T3579: bugfix when deleting non existent iptable rules

We only delete iptables rules if they really exist - if we try to delete a non-
existing rule a PermissionError exception is thrown.

We could either ignore the error code (that is what the old Vyatta code did),
or we check what we are doing beforehand.

vyos/vyos-1x,vyos/vyos-1x,vyos/vyos-1x,vyos/vyos-1x