IAP should route to central ui instead of jupyterhub

[kkasravi]

Fixes #809

• Removed stanza in iap.libsonnet shown below

                        // TODO(ankushagarwal): We should eventually
                        // redirect to the central UI once its ready
                        // See https://github.com/kubeflow/kubeflow/pull/146
                        // Redirect to jupyterhub when visiting /
                        {
                          timeout_ms: 600000,
                          path: "/",
                          prefix_rewrite: "/hub",
                          use_websocket: true,
                          weighted_clusters: {
                            clusters: [
                              {
                                name: "cluster_jupyterhub",
                                weight: 100.0,
                              },
                            ],
                          },
                        },

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This change is 

[kkasravi]

/assign @ankushagarwal

[kkasravi]

/cc @jlewi

[ankushagarwal]

/lgtm
/approve

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