nfs: remove dead code for the old swap over NFS implementation

Remove the code testing folio_test_swapcache either explicitly or
implicitly in pagemap.h headers, as is now handled using the direct I/O
path and not the buffered I/O path that these helpers are located in.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>