cgroup: remove for_each_root_subsys()

After the previous patch which introduced for_each_css(),
for_each_root_subsys() only has two users left.  This patch replaces
it with for_each_subsys() + explicit subsys_mask testing and remove
for_each_root_subsys() along with cgroupfs_root->subsys_list handling.

This patch doesn't introduce any behavior changes.

Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Li Zefan <lizefan@huawei.com>