NFS: Replace old NFSv2 decoder functions with xdr_stream-based ones

Clean up.  Remove unused legacy result decoder functions, and any
now unused decoder helper functions.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Tested-by: J. Bruce Fields <bfields@redhat.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>