mm/slab: remove CONFIG_SLOB code from slab common code

CONFIG_SLOB has been removed from Kconfig. Remove code and #ifdef's
specific to SLOB in the slab headers and common code.

Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
Reviewed-by: Hyeonggon Yoo <42.hyeyoo@gmail.com>
Acked-by: Lorenzo Stoakes <lstoakes@gmail.com>