mm/slub: remove 'per_cpu' which is useless variable

Remove 'per_cpu', since it is useless now after the patch: "205ab99d
slub: Update statistics handling for variable order slabs". And the
partial list is handled in the same way as the per cpu slab.

Acked-by: Christoph Lameter <cl@linux.com>
Signed-off-by: Chen Gang <gang.chen@asianux.com>
Signed-off-by: Pekka Enberg <penberg@kernel.org>