cpuset: Remove set but not used variable 'cs'

Fixes gcc '-Wunused-but-set-variable' warning:

kernel/cgroup/cpuset.c: In function 'cpuset_cancel_attach':
kernel/cgroup/cpuset.c:2167:17: warning:
 variable 'cs' set but not used [-Wunused-but-set-variable]

It never used since introduction in commit 1f7dd3e5 ("cgroup: fix handling
of multi-destination migration from subtree_control enabling")

Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Tejun Heo <tj@kernel.org>