neighbour: Remove unused inline function neigh_key_eq16()

commit c8f01a4a upstream.

All uses of neigh_key_eq16() have
been removed since commit 1202cdd6 ("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>