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/test/samples_index_test.dart

@@ -122,8 +122,8 @@ void main() {
 
       // Test if queries match by type
       expect(matchesQuery('type:sample', attributes), true);
-      expect(matchesQuery('type:cookbook', attributes), false);
-      expect(matchesQuery('kittens type:cookbook', attributes), false);
+      expect(matchesQuery('type:demo', attributes), false);
+      expect(matchesQuery('kittens type:demo', attributes), false);
     });
   });
 
@@ -134,7 +134,7 @@ void main() {
       expect(parseHash('#?search=kittens&platform=web'),
           containsPair('platform', 'web'));
       expect(parseHash('#?type=sample'), containsPair('type', 'sample'));
-      expect(parseHash('#?type=cookbook'), containsPair('type', 'cookbook'));
+      expect(parseHash('#?type=demo'), containsPair('type', 'demo'));
     });
 
     test('can be set', () {

web/samples_index/test/yaml/bad.yaml

@@ -22,6 +22,6 @@ samples:
         href: https://apps.apple.com/us/app/neko-atsume-kitty-collector/id923917775
       - text: author
         href: http://jpryan.me
-    type: sample # sample, app, or cookbook
+    type: sample # sample or app
     date: 2019-12-15T02:59:43.1Z
     channel: stable

web/samples_index/test/yaml/single.yaml

@@ -16,6 +16,6 @@ samples:
       - path
     tags: ['beginner', 'kittens', 'cats']
     platforms: ['web', 'ios', 'android']
-    type: sample # sample, app, or cookbook
+    type: sample # sample or app
     date: 2019-12-15T02:59:43.1Z
     channel: stable