NFS: Remove old NFSv2 encoder functions

Clean up:  Remove unused legacy argument encoder functions, and any
now unused encoder 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>