neigh: remove next ptr from struct neigh_table

After commit
d7480fd3 ("neigh: remove dynamic neigh table registration support"),
this field is not used anymore.

CC: Cong Wang <xiyou.wangcong@gmail.com>
Signed-off-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Acked-by: Cong Wang <xiyou.wangcong@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>