Remove the deprecated "kmem_cache_t" typedef from slab.h.

Given that there is no remaining usage of the deprecated kmem_cache_t
typedef anywhere in the tree, remove that typedef.

Signed-off-by: Robert P. J. Day <rpjday@mindspring.com>
Acked-by: Pekka Enberg <penberg@cs.helsinki.fi>
Acked-by: Christoph Lameter <clameter@sgi.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>