neighbour: Remove NEIGH_DN_TABLE.

Since commit 1202cdd6 ("Remove DECnet support from kernel"),
NEIGH_DN_TABLE is no longer used.

MPLS has implicit dependency on it in nla_put_via(), but nla_get_via()
does not support DECnet.

Let's remove NEIGH_DN_TABLE.

Now, neigh_tables[] has only 2 elements and no extra iteration
for DECnet in many places.

Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Link: https://patch.msgid.link/20241014235216.10785-1-kuniyu@amazon.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>