nfs: Remove writepage

NFS already has writepages and migrate_folio, so it does not need to
implement writepage.  The writepage operation is deprecated as it leads
to worse performance under high memory pressure due to folios being
written out in LRU order rather than sequentially within a file.

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>