mm/mempool/dmapool: remove CONFIG_DEBUG_SLAB ifdefs

CONFIG_DEBUG_SLAB is going away with CONFIG_SLAB, so remove dead ifdefs
in mempool and dmapool code.

Reviewed-by: Kees Cook <keescook@chromium.org>
Acked-by: David Rientjes <rientjes@google.com>
Tested-by: David Rientjes <rientjes@google.com>
Reviewed-by: Hyeonggon Yoo <42.hyeyoo@gmail.com>
Tested-by: Hyeonggon Yoo <42.hyeyoo@gmail.com>
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>