Remove unused struct ifnet's *if_afdata[] and struct domain's dom_if{at,de}tach()

Remove unused struct ifnet's *if_afdata[] and struct domain's dom_if{at,de}tach()

Both made obsolete through struct ifnet's previous *if_nd addition.

IPv6 Neighbour Discovery handles per-interface data directly, nothing
else uses this generic domain API anymore.

Outside of _KERNEL, but nothing in base uses them, either.

OK bluhm mvs claudio