neigh: remove dynamic neigh table registration support

Currently there are only three neigh tables in the whole kernel:
arp table, ndisc table and decnet neigh table. What's more,
we don't support registering multiple tables per family.
Therefore we can just make these tables statically built-in.

Cc: David S. Miller <davem@davemloft.net>
Signed-off-by: Cong Wang <xiyou.wangcong@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

diff --cc include/net/neighbour.h
Simple merge

diff --cc net/core/neighbour.c
Simple merge

diff --cc net/decnet/dn_neigh.c
Simple merge

diff --cc net/ipv4/arp.c
Simple merge

diff --cc net/ipv6/ndisc.c
Simple merge