nilfs2: remove nilfs_writepage

Since nilfs2 has a ->writepages operation already, ->writepage is only
called by the migration code.  If we add a ->migrate_folio operation, it
won't even be used for that and so it can be deleted.

[konishi.ryusuke@gmail.com: fixed panic by using buffer_migrate_folio_norefs]
Link: https://lkml.kernel.org/r/20241002150036.1339475-2-willy@infradead.org
Link: https://lkml.kernel.org/r/20241024092602.13395-10-konishi.ryusuke@gmail.com


Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Signed-off-by: Ryusuke Konishi <konishi.ryusuke@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>