Excluding layer from GoeExt 2 Legend Panel

I'm trying to exclude some legend in GeoExt.panel.Legend.

I've created a vector layer named 'City Extent' via a function but I want the legend to be removed in the list of legends in my legend panel.

The 'filter' option doesn't seem to work

Here's my vector layer for City Extent

city_ext = new OpenLayers.Layer.Vector("City Extent", {
                strategies: [new OpenLayers.Strategy.Fixed()],
                protocol: new OpenLayers.Protocol.WFS({
                    url: geoserverURL,
                    version: "1.1.0",
                    featureType: "gadm2",
                    featureNS: "http://ift.tt/1IAEmHR",
                    srsName: "EPSG:900913"
                }),
                filter: new OpenLayers.Filter.Logical({
                    type: OpenLayers.Filter.Logical.AND,
                    filters: [
                    new OpenLayers.Filter.Comparison({
                        type: OpenLayers.Filter.Comparison.EQUAL_TO,
                        property: "id_2", // city
                        value: params_2
                    }),
                    new OpenLayers.Filter.Comparison({
                        type: OpenLayers.Filter.Comparison.EQUAL_TO,
                        property: "id_1", // province
                        value: params_1
                    }),
                    new OpenLayers.Filter.Comparison({
                        type: OpenLayers.Filter.Comparison.EQUAL_TO,
                        property: "id_0", // country
                        value: params_0
                    })
                ]
                }),
                eventListeners: {
                    'loadend': function(evt) {
                        map.zoomToExtent(city_ext.getDataExtent());
                    }
                }
            });

Here's my GeoExt Legend Panel

var legendPanel = Ext.create('GeoExt.panel.Legend', {
            defaults: {
                labelCls: 'mylabel',
                style: 'padding:5px'
            },
            bodyStyle: 'padding:5px',
            width: 250,
            autoScroll: true,
            layerStore: mapPanel.layers,
            // dynamic: false,
            loader: {
                filter: function(record) {
                    return record.get("layer").name.indexOf("City Extent") == -1
                }
            }
        });

Could someone point to me the proper way to exclude/remove the legend?