nfs: Remove obsoleted declaration for nfs_read_prepare

The nfs_read_prepare() have been removed since
commit a4cdda59 ("NFS: Create a common pgio_rpc_prepare function"),
and now it is useless, so remove it.
Signed-off-by: Gaosheng Cui <cuigaosheng1@huawei.com>
Signed-off-by: Anna Schumaker <anna.schumaker@oracle.com>

fs/nfs/internal.h

@@ -505,7 +505,6 @@ extern int nfs_read_add_folio(struct nfs_pageio_descriptor *pgio,
 			       struct nfs_open_context *ctx,
 			       struct folio *folio);
 extern void nfs_pageio_complete_read(struct nfs_pageio_descriptor *pgio);
-extern void nfs_read_prepare(struct rpc_task *task, void *calldata);
 extern void nfs_pageio_reset_read_mds(struct nfs_pageio_descriptor *pgio);
 
 /* super.c */