Remove unused code under `screens/` (#9911)
* Add check-unused-code to DCM CI checks.
* exclude public API
* Add excludes
* exclude test/
* exclude service/
* exclude integration_test/
* Ignore false positives in app.dart.
* Remove unused code in extensions/ and framework/
* use if (kDebugMode)
* remove other assert
* Remove unused code from `screens/`
* fix test
* Fix bug and format.
