Do not update the lagg link layer address when destroying a lagg clone.

This would enqueue an event to send the gratuitous arp on a dying lagg
interface without any physical ports attached to it.

Apart from that, the taskqueue_drain() on lagg_clone_destroy() runs too
late, when the ifp data structure is already freed. Fix that too.

Obtained from: pfSense
MFC after: 2 weeks
Sponsored by: Rubicon Communications, LLC (Netgate)