Drop retired Flutter Developer Google group (#2103)

## 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

README.md

@@ -37,7 +37,6 @@ See the [contributor's guide](CONTRIBUTING.md)!
 If you have a general question about one of these samples or how to adapt its
 techniques for one of your own apps, try one of these resources:
 
-* [The FlutterDev Google Group](https://groups.google.com/forum/#!forum/flutter-dev)
 * [The FlutterDev Discord](https://discord.gg/rflutterdev)
 * [StackOverflow](https://stackoverflow.com/questions/tagged/flutter)