NFS: Remove nfs_release()

And call nfs_file_clear_open_context() directly.  This makes it obvious
that nfs_file_release() will always return 0.

Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>
Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>