filemap: remove find_subpage()

All users of this function now call folio_file_page() instead.  Delete it.

Link: https://lkml.kernel.org/r/20250402210612.2444135-6-willy@infradead.org


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