lagg: don't update link layer addresses on destroy

When the lagg is being destroyed it is not necessary update the
lladdr of all the lagg members every time we update the primary
interface.

Reviewed by: scottl
Obtained from: pfSense
MFC after: 1 week
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D31586