Remove flights example

sky/benchmarks/framework/city-list.sky

-<sky>
-<import src="/sky/benchmarks/resources/runner.sky" as="PerfRunner" />
-<import src="/sky/examples/city-list/city-list.sky" as="citylist" />
-<script>
-var sky = document.querySelector("sky");
-var element;
-
-var CityListElement = citylist.CityListElement;
-
-var runner = new PerfRunner({
-  setup: function() {
-    // Don't benchmark removal.
-    if (element)
-      element.remove();
-  },
-  iterations: 10,
-  unit: "ms",
-});
-
-runner.runAsync(function(done) {
-  element = new CityListElement();
-  sky.appendChild(element);
-  element.addEventListener('load', function() {
-    done();
-  });
-});
-</script>
-</sky>