Simplify samples index and remove cookbook recipes (#2102)

Remove the cookbook recipes from the samples index in a step to
eventually remove it as a whole.

The cookbook recipe listings in the index haven't been updated in a long
time, the support for updating them doesn't work, and this isn't
generally how people are finding cookbook recipes.

This has the added benefit of reducing repo size quite a bit due to the
large images.

web/samples_index/lib/samples_index.dart

@@ -2,7 +2,6 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file
 
-import 'dart:convert';
 import 'dart:io';
 import 'package:checked_yaml/checked_yaml.dart';
 
@@ -12,14 +11,10 @@ export 'src/data.dart';
 
 Future<List<Sample>> getSamples() async {
   var yamlFile = File('lib/src/samples.yaml');
-  var cookbookFile = File('lib/src/cookbook.json');
   var contents = await yamlFile.readAsString();
-  var cookbookContents = await cookbookFile.readAsString();
   var index = checkedYamlDecode(
       contents, (m) => m != null ? Index.fromJson(m) : null,
       sourceUrl: yamlFile.uri);
   if (index == null) throw ('unable to get load from ${yamlFile.uri}');
-  var cookbookIndex =
-      Index.fromJson(json.decode(cookbookContents) as Map<dynamic, dynamic>);
-  return index.samples..addAll(cookbookIndex.samples);
+  return index.samples;
 }