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>