filemap: remove readahead_page_batch()

This function has no more callers; delete it.

Link: https://lkml.kernel.org/r/20250402210612.2444135-8-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>