NFS: remove unused struct 'mnt_fhstatus'

'mnt_fhstatus' has been unused since
commit 065015e5 ("NFS: Remove unused XDR decoder functions").

Remove it.

Signed-off-by: Dr. David Alan Gilbert <linux@treblig.org>
Reviewed-by: Benjamin Coddington <bcodding@redhat.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>