Revert "cgroup: add cgroup_subsys->css_e_css_changed()"

This reverts commit 56c807ba.

cgroup_subsys->css_e_css_changed() was supposed to be used by cgroup
writeback support; however, the change to per-inode cgroup association
made it unnecessary and the callback doesn't have any user.  Remove
it.
Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>

include/linux/cgroup-defs.h

@@ -434,7 +434,6 @@ struct cgroup_subsys {
 	void (*css_released)(struct cgroup_subsys_state *css);
 	void (*css_free)(struct cgroup_subsys_state *css);
 	void (*css_reset)(struct cgroup_subsys_state *css);
-	void (*css_e_css_changed)(struct cgroup_subsys_state *css);
 
 	int (*can_attach)(struct cgroup_taskset *tset);
 	void (*cancel_attach)(struct cgroup_taskset *tset);

kernel/cgroup.c

@@ -3127,24 +3127,6 @@ static ssize_t cgroup_subtree_control_write(struct kernfs_open_file *of,
 		}
 	}
 
-	/*
-	 * The effective csses of all the descendants (excluding @cgrp) may
-	 * have changed.  Subsystems can optionally subscribe to this event
-	 * by implementing ->css_e_css_changed() which is invoked if any of
-	 * the effective csses seen from the css's cgroup may have changed.
-	 */
-	for_each_subsys(ss, ssid) {
-		struct cgroup_subsys_state *this_css = cgroup_css(cgrp, ss);
-		struct cgroup_subsys_state *css;
-
-		if (!ss->css_e_css_changed || !this_css)
-			continue;
-
-		css_for_each_descendant_pre(css, this_css)
-			if (css != this_css)
-				ss->css_e_css_changed(css);
-	}
-
 	kernfs_activate(cgrp->kn);
 	ret = 0;
 out_unlock: