Mistake in documentation for MAGNUM_AVAILABLE_ADDONS configuration

The examples values given for MAGNUM_AVAILABLE_ADD_ONS include a 'labels_unselected' data structure.

'labels_unselected' is not implemented. Including in the configuration will have no affect or consequences it will be completely ignored.

It should be removed from the documentation.

https://docs.openstack.org/magnum-ui/latest/configuration/index.html#magnum-available-addons
```
MAGNUM_AVAILABLE_ADDONS = [
    {
        "name": "Kubernetes Dashboard",
        "selected": True,
        "labels": {
            "kube_dashboard_enabled": True
        },
        "labels_unselected": {
            "kube_dashboard_enabled": False
        }
    },