Remove mirroring workflow. (#1619)

The master to main branch migration has been successful with main branch
being used for a few months already.

Bug: flutter/flutter#90476


## Pre-launch Checklist

- [X] I read the [Effective Dart: Style] _recently_, and have followed
its advice.
- [X] I signed the [CLA].
- [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 -->
[Effective Dart: Style]:
https://dart.dev/guides/language/effective-dart/style
[CLA]: https://cla.developers.google.com/
[Discord]: https://github.com/flutter/flutter/wiki/Chat