mm,memory_hotplug: remove status_change_nid_normal and update documentation

mm,memory_hotplug: remove status_change_nid_normal and update documentation

Now that the last user of status_change_nid_normal is gone, we can remove
it.  Update documentation accordingly.

Link: https://lkml.kernel.org/r/20250616135158.450136-3-osalvador@suse.de
Signed-off-by: Oscar Salvador <osalvador@suse.de>
Reviewed-by: Vlastimil Babka <vbabka@suse.cz>
Acked-by: David Hildenbrand <david@redhat.com>
Cc: Harry Yoo <harry.yoo@oracle.com>
Cc: Hyeonggon Yoo <42.hyeyoo@gmail.com>
Cc: Joanthan Cameron <Jonathan.Cameron@huawei.com>
Cc: Rakie Kim <rakie.kim@sk.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>