sched/psi: Remove redundant cgroup_psi() when !CONFIG_CGROUPS

cgroup_psi() is only called under CONFIG_CGROUPS.
We don't need cgroup_psi() when !CONFIG_CGROUPS,
so we can remove it in this case.
Signed-off-by: Hao Jia <jiahao.os@bytedance.com>
Reviewed-by: Ingo Molnar <mingo@kernel.org>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Signed-off-by: Tejun Heo <tj@kernel.org>

include/linux/cgroup.h

@@ -734,11 +734,6 @@ static inline struct cgroup *cgroup_parent(struct cgroup *cgrp)
 	return NULL;
 }
 
-static inline struct psi_group *cgroup_psi(struct cgroup *cgrp)
-{
-	return NULL;
-}
-
 static inline bool cgroup_psi_enabled(void)
 {
 	return false;