cgroup-util: drop unused cg_rmdir()

When removing a cgroup, we always want to eliminate subcgroups
first, i.e. use cg_trim(). And cg_rmdir() (along with
CGROUP_REMOVE flag) is simply unused. Kill it.

src/shared/cgroup-setup.c

@@ -917,12 +917,6 @@ int cg_migrate_recursive(
         if (r < 0 && ret >= 0)
                 ret = r;
 
-        if (flags & CGROUP_REMOVE) {
-                r = cg_rmdir(cfrom, pfrom);
-                if (r < 0 && ret >= 0 && !IN_SET(r, -ENOENT, -EBUSY))
-                        return r;
-        }
-
         return ret;
 }