sfxge(4): remove obsolete Wake-On-LAN support

Wake-on-lan is not supported in production on any of our adapters, as
they don't have the required AUX power connector. (It's possible that
AUX power is supplied to some of our ALOM or mezz adapters, but if so
then we've never implemented or tested WoL support.)

Reviewed by: gnn
Sponsored by: Solarflare Communications, Inc.
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D8972