Remove Gallery from web index (#2464)

Remove Flutter Gallary from the visual index at
https://flutter.github.io/samples/, as the Gallery is no longer being
maintained

## 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/blob/master/docs/contributing/Style-guide-for-Flutter-repo.md
[CLA]: https://cla.developers.google.com/
[Discord]:
https://github.com/flutter/flutter/blob/master/docs/contributing/Chat.md
[Contributors Guide]:
https://github.com/flutter/samples/blob/main/CONTRIBUTING.md

web/samples_index/lib/src/samples.yaml

@@ -52,92 +52,6 @@ samples:
     type: demo
     web: web/simplistic_editor
 
-  - name: Gallery
-    author: Flutter
-    screenshots:
-      - url: images/gallery1.png
-        alt: Gallery app screenshot
-      - url: images/gallery2.png
-        alt: Rally app screenshot
-      - url: images/gallery3.png
-        alt: Fortnightly app screenshot
-      - url: images/gallery4.png
-        alt: Crane app screenshot
-      - url: images/gallery5.png
-        alt: Shrine app screenshot
-    source: https://github.com/flutter/gallery
-    web: https://flutter-gallery-archive.web.app
-    description: >
-      A collection of Material Design & Cupertino widgets, behaviors,
-      and vignettes implemented with Flutter.
-    difficulty: intermediate
-    widgets:
-      - AlertDialog
-      - AppBar
-      - BottomAppBar
-      - BottomNavigationBar
-      - BottomSheet
-      - Card
-      - Checkbox
-      - ChoiceChip
-      - CircularProgressIndicator
-      - Container
-      - CupertinoActivityIndicator
-      - CupertinoAlertDialog
-      - CupertinoButton
-      - CupertinoButton
-      - CupertinoDatePicker
-      - CupertinoDialogAction
-      - CupertinoNavigationBar
-      - CupertinoPageScaffold
-      - CupertinoSegmentedControl
-      - CupertinoSlider
-      - CupertinoSlidingSegmentedControl
-      - CupertinoSliverRefreshControl
-      - CupertinoSwitch
-      - CupertinoTabView
-      - CupertinoTextField
-      - CupertinoTheme
-      - DayPicker
-      - FilterChip
-      - FlatButton
-      - FloatingActionButton
-      - GridTile
-      - GridView
-      - Icon
-      - InputChip
-      - LayoutBuilder
-      - LinearProgressIndicator
-      - ListTile
-      - ListView
-      - MaterialBanner
-      - MonthPicker
-      - PaginatedDataTable
-      - PopupMenuButton
-      - PopupMenuItem
-      - Radio
-      - RaisedButton
-      - RangeSlider
-      - Scaffold
-      - SimpleDialog
-      - Slider
-      - SnackBar
-      - Switch
-      - TabBar
-      - TabBarView
-      - TextField
-      - TextFormField
-      - Tooltip
-      - YearPicker
-    packages:
-      - flutter/material
-      - flutter/cupertino
-      - google_fonts
-      - scoped_model
-    tags: ['intermediate', 'sample', 'gallery', 'material', 'design', 'vignettes']
-    platforms: ['web', 'ios', 'android']
-    type: demo
-
   - name: Web Embedding
     author: Flutter and Angular
     screenshots: