cgroup: move subsys file removal to kill_css()

With the planned unified hierarchy, individual css's will be created
and destroyed dynamically across the lifetime of a cgroup.  To enable
such usages, css destruction is being decoupled from cgroup
destruction.  This patch moves subsys file removal from
cgroup_destroy_locked() to kill_css().

While this changes the order of destruction operations, the changes
shouldn't be noticeable to cgroup subsystems or userland.

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