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].

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[Flutter Style Guide]:
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[Contributors Guide]:
https://github.com/flutter/samples/blob/main/CONTRIBUTING.md

web/samples_index/lib/src/data.dart

@@ -68,9 +68,6 @@ class Sample {
   /// 'web'
   final List<String> platforms;
 
-  /// Links to display on the details page
-  final List<Link> links;
-
   /// The type of the sample. Supported values are either 'sample' or
   /// 'cookbook'.
   final String type;
@@ -94,7 +91,6 @@ class Sample {
     this.packages = const [],
     this.tags = const [],
     this.platforms = const [],
-    this.links = const [],
     required this.type,
     this.date,
     this.channel,
@@ -172,16 +168,3 @@ class Screenshot {
 
   Map<String, dynamic> toJson() => _$ScreenshotToJson(this);
 }
-
-/// An external link displayed next to a sample
-@JsonSerializable(anyMap: true, checked: true)
-class Link {
-  final String text;
-  final String href;
-
-  Link(this.text, this.href);
-
-  factory Link.fromJson(Map<dynamic, dynamic> json) => _$LinkFromJson(json);
-
-  Map<String, dynamic> toJson() => _$LinkToJson(this);
-}