slab: remove a useless lockdep annotation

Now, there is no code to hold two lock simultaneously, since we don't
call slab_destroy() with holding any lock.  So, lockdep annotation is
useless now.  Remove it.

v2: don't remove BAD_ALIEN_MAGIC in this patch. It will be removed
    in the following patch.

Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Acked-by: Christoph Lameter <cl@linux.com>
Cc: Pekka Enberg <penberg@kernel.org>
Cc: David Rientjes <rientjes@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>