cgroup: remove unnecessary empty check when enabling threaded mode

cgroup_enable_threaded() checks that the cgroup doesn't have any tasks
or children and fails the operation if so.  This test is unnecessary
because the first part is already checked by
cgroup_can_be_thread_root() and the latter is unnecessary.  The latter
actually cause a behavioral oddity.  Please consider the following
hierarchy.  All cgroups are domains.

    A
   / \
  B   C
       \
        D

If B is made threaded, C and D becomes invalid domains.  Due to the no
children restriction, threaded mode can't be enabled on C.  For C and
D, the only thing the user can do is removal.

There is no reason for this restriction.  Remove it.

Acked-by: Waiman Long <longman@redhat.com>
Signed-off-by: Tejun Heo <tj@kernel.org>