mm: remove stale example from comment

folio_add_new_anon_rmap() no longer works this way, so just remove the
entire example.

Link: https://lkml.kernel.org/r/20231211162214.2146080-6-willy@infradead.org


Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: David Hildenbrand <david@redhat.com>
Cc: Ralph Campbell <rcampbell@nvidia.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>