stop fetching dart tags and remove old recipes

We have not needed to fetch Dart tags since
https://dart-review.googlesource.com/c/sdk/+/155681
was merged.

Bug: https://github.com/flutter/flutter/issues/67966
Change-Id: If9f8b28feb1ed25355ea94e04afe28f6432a9db9
Reviewed-on: https://flutter-review.googlesource.com/c/recipes/+/7620
Commit-Queue: Christopher Fujino <fujino@google.com>
Reviewed-by: Godofredo Contreras <godofredoc@google.com>
Reviewed-by: Keyong Han <keyonghan@google.com>