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>