NFS: Revert creation of IPv6 listeners for lockd and NFSv4 callbacks

We're about to convert over to using separate PF_INET and PF_INET6
listeners, instead of a single PF_INET6 listener that also receives
AF_INET requests and maps them to AF_INET6.

Clear the way by removing the logic in lockd and the NFSv4 callback
server that creates an AF_INET6 service listener.

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