NFS: Remove support for the 'mountprog' option

Remove the mount option that allows users to specify an alternate mountd
program number.  The client hasn't support setting an alternate mountd
program number for a very long time.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>