web/samples_index Drop unused links attribute from config (#1828)

I've confirmed that the `links` configuration attribute is indeed
unused: https://flutter.github.io/samples/next_gen_u_i_demo.html

Dropping unused configuration sounds like a good move. We can
re-introduce it if/when we start displaying the content.

## Pre-launch Checklist

- [x] I read the [Flutter Style Guide] _recently_, and have followed its
advice.
- [x] I signed the [CLA].
- [x] I read the [Contributors Guide].
- [x] I updated/added relevant documentation (doc comments with `///`).
- [x] All existing and new tests are passing.

If you need help, consider asking for advice on the #hackers-devrel
channel on [Discord].

<!-- Links -->
[Flutter Style Guide]:
https://github.com/flutter/flutter/wiki/Style-guide-for-Flutter-repo
[CLA]: https://cla.developers.google.com/
[Discord]: https://github.com/flutter/flutter/wiki/Chat
[Contributors Guide]:
https://github.com/flutter/samples/blob/main/CONTRIBUTING.md

web/samples_index/test/samples_index_test.dart

@@ -38,9 +38,6 @@ void main() {
       expect(sample.tags, hasLength(3));
       expect(sample.tags[1], 'kittens');
       expect(sample.platforms, hasLength(3));
-      expect(sample.links, hasLength(2));
-      expect(sample.links[1].text, 'author');
-      expect(sample.links[1].href, 'https://jpryan.me');
       expect(sample.type, 'sample');
       expect(sample.date, DateTime.parse('2019-12-15T02:59:43.1Z'));
       expect(sample.channel, 'stable');

web/samples_index/test/yaml/single.yaml

@@ -16,11 +16,6 @@ samples:
       - path
     tags: ['beginner', 'kittens', 'cats']
     platforms: ['web', 'ios', 'android']
-    links:
-      - text: inspiration
-        href: https://apps.apple.com/us/app/neko-atsume-kitty-collector/id923917775
-      - text: author
-        href: https://jpryan.me
     type: sample # sample, app, or cookbook
     date: 2019-12-15T02:59:43.1Z
     channel: stable