memory: remove unused register_hotmemory_notifier()

Remove unused register_hotmemory_notifier().

Link: https://lkml.kernel.org/r/20220923033347.3935160-8-liushixin2@huawei.com


Signed-off-by: Liu Shixin <liushixin2@huawei.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Cc: Christoph Lameter <cl@linux.com>
Cc: Kefeng Wang <wangkefeng.wang@huawei.com>
Cc: Waiman Long <longman@redhat.com>
Cc: zefan li <lizefan.x@bytedance.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>