NFSv4: cleanup remove unused nfs4_xdev_fs_type

commit e8f25e6d "NFS: Remove the NFS v4 xdev mount function"
removed the last use of this.

Signed-off-by: Olga Kornievskaia <kolga@netapp.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>