sfxge: remove obsolete efx_mac_hash_set() from common code

This API has been replaced by efx_mac_multicast_list_set()
and has no callers.

Submitted by: Andy Moreton <amoreton at solarflare.com>
Reviewed by: gnn
Sponsored by: Solarflare Communications, Inc.
MFC after: 2 days
Differential Revision: https://reviews.freebsd.org/D4883