cpuset: remove fast exit path from remove_tasks_in_empty_cpuset()

The function isn't that hot, the overhead of missing the fast exit is
low, the test itself depends heavily on cgroup internals, and it's
gonna be a hindrance when trying to decouple cpuset locking from
cgroup core.  Remove the fast exit path.

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