cgroup: remove cgroup_subsys->populate()

With memcg converted, cgroup_subsys->populate() doesn't have any user
left.  Remove it.
Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Li Zefan <lizefan@huawei.com>

include/linux/cgroup.h

@@ -472,7 +472,6 @@ struct cgroup_subsys {
 	void (*fork)(struct task_struct *task);
 	void (*exit)(struct cgroup *cgrp, struct cgroup *old_cgrp,
 		     struct task_struct *task);
-	int (*populate)(struct cgroup_subsys *ss, struct cgroup *cgrp);
 	void (*post_clone)(struct cgroup *cgrp);
 	void (*bind)(struct cgroup *root);
 

kernel/cgroup.c

@@ -3842,9 +3842,6 @@ static int cgroup_populate_dir(struct cgroup *cgrp)
 	for_each_subsys(cgrp->root, ss) {
 		struct cftype_set *set;
 
-		if (ss->populate && (err = ss->populate(ss, cgrp)) < 0)
-			return err;
-
 		list_for_each_entry(set, &ss->cftsets, node)
 			cgroup_addrm_files(cgrp, ss, set->cfts, true);
 	}