neighbour: Remove unused inline function neigh_key_eq16()
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>