NFS: Remove rpcbind cleanup for NFSv4.0 callback

The NFS client's NFSv4.0 callback listeners are created with
SVC_SOCK_ANONYMOUS, therefore svc_setup_socket() does not register
them with the client's rpcbind service.

And, note that nfs_callback_down_net() does not call
svc_rpcb_cleanup() at all when shutting down the callback server.

Even if svc_setup_socket() were to attempt to register or unregister
these sockets, the callback service has vs_hidden set, which shunts
the rpcbind upcalls.

The svc_rpcb_cleanup() error flow was introduced by
commit c946556b ("NFS: move per-net callback thread
initialization to nfs_callback_up_net()"). It doesn't appear in the
code that was relocated by that commit.

Therefore, there is no need to call svc_rpcb_cleanup() when listener
creation fails during callback server start-up.

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