mm/vmstat: remove unused node and zone state helpers

mm/vmstat: remove unused node and zone state helpers

Several helper functions for managing node and zone states have become
obsolete and no longer have any callers within the kernel.

  inc_node_state()
  inc_zone_state()
  dec_zone_state()

This commit removes the dead code.

Link: https://lkml.kernel.org/r/20251225210213.2553-1-richard.weiyang@gmail.com
Signed-off-by: Wei Yang <richard.weiyang@gmail.com>
Reviewed-by: Joshua Hahn <joshua.hahnjy@gmail.com>
Acked-by: David Hildenbrand (Red Hat) <david@kernel.org>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>