neighbour: Remove unused inline function neigh_key_eq16()

stable inclusion
from stable-v5.10.185
commit 53076071fb92a46119ba7f4e3369f1e7a5237f9c
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I8L5XP

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=53076071fb92a46119ba7f4e3369f1e7a5237f9c



--------------------------------

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>
Signed-off-by: sanglipeng <sanglipeng1@jd.com>