Remove visual samples index (#2607)

Removes VSI and all related infra. It has been replaced by [an index
page on
flutter.dev](https://docs.flutter.dev/reference/learning-resources)

Fixes:

#2582 
#2507 

## 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 have added sample code updates to the [changelog].
- [x] I updated/added relevant documentation (doc comments with `///`).

.github/workflows/main.yml

@@ -37,17 +37,6 @@ jobs:
           channel: ${{ matrix.flutter_version }}
       - run: ./tool/flutter_ci_script_${{ matrix.flutter_version }}.sh
 
-  web-samples-index:
-    name: web/samples_index config check
-    runs-on: ubuntu-latest
-    steps:
-      - uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683
-      - uses: subosito/flutter-action@e938fdf56512cc96ef2f93601a5a40bde3801046
-      - run: |
-          dart pub get
-          dart run grinder generate
-        working-directory: web/samples_index
-
   # android-build:
   #   runs-on: ubuntu-latest
   #   if: github.repository == 'flutter/samples'