Remove custom elements examples, they are no longer maintained

sky/examples/scrolling/scrolling.sky

-<!--
-// Copyright 2015 The Chromium Authors. All rights reserved.
-// Use of this source code is governed by a BSD-style license that can be
-// found in the LICENSE file.
--->
-<import src="/sky/framework/elements/sky-element.sky" />
-<import src="/sky/framework/elements/sky-scrollable.sky" />
-
-<sky-element>
-<template>
-  <style>
-  sky-scrollable {
-    margin: 20px;
-    height: 400px;
-    border: 2px solid blue;
-  }
-  </style>
-  <sky-scrollable />
-</template>
-<script>
-import "dart:sky";
-import "../data/cities.dart" as cities;
-
-@Tagname('scrolling')
-class Scrolling extends SkyElement {
-  void shadowRootReady() {
-    Element parent = shadowRoot.querySelector('sky-scrollable');
-    for (Map city in cities.kData.getRange(0, 300)) {
-      Element div = document.createElement('div');
-      div.setChild(new Text(city['name']));
-      parent.appendChild(div);
-    }
-  }
-}
-
-_init(script) => register(script, Scrolling);
-</script>
-</sky-element>