affs: remove writepage implementation

If the filesystem implements migrate_folio and writepages, there is
no need for a writepage implementation.

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>