mm/slub: remove dead code for debug caches on deactivate_slab()

After commit c7323a5a ("mm/slub: restrict sysfs validation to debug
caches and make it safe"), SLUB never installs percpu slab for debug caches
and thus never deactivates percpu slab for them.

Since only debug caches use the full list, SLUB no longer deactivates to
full list. Remove dead code in deactivate_slab().

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