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

kernel/cgroup/cpuset.c

@@ -2109,10 +2109,8 @@ static int cpuset_can_attach(struct cgroup_taskset *tset)
 static void cpuset_cancel_attach(struct cgroup_taskset *tset)
 {
 	struct cgroup_subsys_state *css;
-	struct cpuset *cs;
 
 	cgroup_taskset_first(tset, &css);
-	cs = css_cs(css);
 
 	mutex_lock(&cpuset_mutex);
 	css_cs(css)->attach_in_progress--;