SLUB: Remove depends on EXPERIMENTAL and !ARCH_USES_SLAB_PAGE_STRUCT

No arch sets ARCH_USES_SLAB_PAGE_STRUCT anymore.

Remove the experimental dependency as well since we want to have it as
a real alternative to SLAB.

It all comes down to killing a single line from init/Kconfig.

Signed-off-by: Christoph Lameter <clameter@sgi.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>