mm: cleanup WORKINGSET_NODES in workingset

WORKINGSET_NODES is not exposed in the memcg stats and thus there is no
need to use the memcg specific stat update functions for it.  In future if
we decide to expose WORKINGSET_NODES in the memcg stats, we can revert
this patch.

Link: https://lkml.kernel.org/r/20240501172617.678560-7-shakeel.butt@linux.dev


Signed-off-by: Shakeel Butt <shakeel.butt@linux.dev>
Reviewed-by: Roman Gushchin <roman.gushchin@linux.dev>
Reviewed-by: T.J. Mercier <tjmercier@google.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Muchun Song <muchun.song@linux.dev>
Cc: Yosry Ahmed <yosryahmed@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>