neighbour: Remove bare neighbour::next pointer

Remove the now-unused neighbour::next pointer, leaving struct neighbour
solely with the hlist_node implementation.

Signed-off-by: Gilad Naaman <gnaaman@drivenets.com>
Reviewed-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Link: https://patch.msgid.link/20241107160444.2913124-6-gnaaman@drivenets.com


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