mm/page-flags: remove folio_start_writeback_keepwrite()

mm/page-flags: remove folio_start_writeback_keepwrite()

Commit cd57b77197a4 ("ext4: Convert ext4_bio_write_page() to use a folio)
removed set_page_writeback_keepwrite() which was the last/only caller of
folio_start_writeback_keepwrite().

Link: https://lkml.kernel.org/r/20250722182230.2114587-1-joannelkoong@gmail.com
Signed-off-by: Joanne Koong <joannelkoong@gmail.com>
Reviewed-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>