nfs: remove unused header <linux/pnfs_osd_xdr.h>

Commit 19fcae3 ("scsi: remove the SCSI OSD library") deleted the last
file that included <linux/pnfs_osd_xdr.h> but left that file behind.  It's
unused, get rid of it now.

Cc: Christoph Hellwig <hch@lst.de>
Cc: Trond Myklebust <trond.myklebust@hammerspace.com>
Cc: Anna Schumaker <anna.schumaker@netapp.com>
Cc: linux-nfs@vger.kernel.org
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>