Remove the redundant drop-down button demo (#4380)

Turns out we have the same demo in the "Buttons" section now.
We don't need both.

examples/flutter_gallery/lib/demo/all.dart

@@ -11,7 +11,6 @@ export 'colors_demo.dart';
 export 'data_table_demo.dart';
 export 'date_picker_demo.dart';
 export 'dialog_demo.dart';
-export 'drop_down_demo.dart';
 export 'grid_list_demo.dart';
 export 'icons_demo.dart';
 export 'leave_behind_demo.dart';

examples/flutter_gallery/lib/demo/drop_down_demo.dart

-// 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 'package:flutter/material.dart';
-
-class DropDownDemo extends StatefulWidget {
-  static const String routeName = '/dropdown';
-
-  @override
-  _DropDownDemoState createState() => new _DropDownDemoState();
-}
-
-class _DropDownDemoState extends State<DropDownDemo> {
-  String _value = "Free";
-
-  List<DropDownMenuItem<String>> buildItems() {
-    return <String>["One", "Two", "Free", "Four"].map((String value) {
-      return new DropDownMenuItem<String>(value: value, child: new Text(value));
-    })
-    .toList();
-  }
-
-  @override
-  Widget build(BuildContext context) {
-    return new Scaffold(
-      appBar: new AppBar(title: new Text('Drop-down button')),
-      body: new Center(
-        child: new DropDownButton<String>(
-          items: buildItems(),
-          value: _value,
-          onChanged: (String newValue) {
-            setState(() {
-              if (newValue != null)
-                _value = newValue;
-            });
-          }
-        )
-      )
-    );
-  }
-}

examples/flutter_gallery/lib/gallery/app.dart

@@ -20,7 +20,6 @@ final Map<String, WidgetBuilder> kRoutes = <String, WidgetBuilder>{
   DatePickerDemo.routeName: (BuildContext context) => new DatePickerDemo(),
   DataTableDemo.routeName: (BuildContext context) => new DataTableDemo(),
   DialogDemo.routeName: (BuildContext context) => new DialogDemo(),
-  DropDownDemo.routeName: (BuildContext context) => new DropDownDemo(),
   TwoLevelListDemo.routeName: (BuildContext context) => new TwoLevelListDemo(),
   TabsFabDemo.routeName: (BuildContext context) => new TabsFabDemo(),
   GridListDemo.routeName: (BuildContext context) => new GridListDemo(),

examples/flutter_gallery/lib/gallery/home.dart

@@ -87,7 +87,6 @@ class GalleryHomeState extends State<GalleryHome> {
               new GalleryItem(title: 'Date picker', routeName: DatePickerDemo.routeName),
               new GalleryItem(title: 'Data tables', routeName: DataTableDemo.routeName),
               new GalleryItem(title: 'Dialog', routeName: DialogDemo.routeName),
-              new GalleryItem(title: 'Drop-down button', routeName: DropDownDemo.routeName),
               new GalleryItem(title: 'Expand/collapse list control', routeName: TwoLevelListDemo.routeName),
               new GalleryItem(title: 'Floating action button', routeName: TabsFabDemo.routeName),
               new GalleryItem(title: 'Grid', routeName: GridListDemo.routeName),