mm, slab: remove memcg_from_slab_obj()

This empty wrapped exists only for !CONFIG_MEMCG_KMEM and seems it was
never used. Probably a leftover from development of a series.
Reviewed-by: Chengming Zhou <chengming.zhou@linux.dev>
Reviewed-by: Roman Gushchin <roman.gushchin@linux.dev>
Acked-by: David Rientjes <rientjes@google.com>
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>

mm/slub.c

@@ -2028,11 +2028,6 @@ void memcg_slab_alloc_error_hook(struct kmem_cache *s, int objects,
 		obj_cgroup_uncharge(objcg, objects * obj_full_size(s));
 }
 #else /* CONFIG_MEMCG_KMEM */
-static inline struct mem_cgroup *memcg_from_slab_obj(void *ptr)
-{
-	return NULL;
-}
-
 static inline void memcg_free_slab_cgroups(struct slab *slab)
 {
 }