neighbour: Remove unused inline function neigh_key_eq16()

commit c8f01a4a54473f88f8cc0d9046ec9eb5a99815d5 upstream.

All uses of neigh_key_eq16() have
been removed since commit 1202cdd66531 ("Remove DECnet support
from kernel"), so remove it.

Signed-off-by: Gaosheng Cui <cuigaosheng1@huawei.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>