memcg: remove duplication detection for mem_cgroup_uncharge_swap

__mem_cgroup_uncharge_swap is only called in mem_cgroup_uncharge_swap, if
mem cgroup is disabled, __mem_cgroup_uncharge_swap cannot be called. 
Therefore, there is no need to judge whether mem_cgroup is disabled or
not.

Link: https://lkml.kernel.org/r/20230819081302.1217098-1-lujialin4@huawei.com


Signed-off-by: Lu Jialin <lujialin4@huawei.com>
Acked-by: Shakeel Butt <shakeelb@google.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Roman Gushchin <roman.gushchin@linux.dev>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Muchun Song <songmuchun@bytedance.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>