cpuset: fix compile warning when CONFIG_SMP=n

Reported by Fengguang's kbuild test robot:

kernel/cpuset.c:787: warning: 'generate_sched_domains' defined but not used

Introduced by commit e0e80a02
("cpuset: use rebuild_sched_domains() in cpuset_hotplug_workfn()),
which removed generate_sched_domains() from cpuset_hotplug_workfn().
Reported-by: Fengguang Wu <fengguang.wu@intel.com>
Signed-off-by: Li Zefan <lizefan@huawei.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

kernel/cpuset.c

@@ -789,13 +789,6 @@ static void rebuild_sched_domains_locked(void)
 static void rebuild_sched_domains_locked(void)
 {
 }
-
-static int generate_sched_domains(cpumask_var_t **domains,
-			struct sched_domain_attr **attributes)
-{
-	*domains = NULL;
-	return 1;
-}
 #endif /* CONFIG_SMP */
 
 void rebuild_sched_domains(void)