remove Wizard.done()

[d-loose]

See canonical/ubuntu-desktop-installer#1879

[jpnurmi]

Presumably, we can remove all of these too:

$ git grep -i done
lib/src/route.dart:/// The signature of [WizardRoute.onDone] callback.
lib/src/route.dart:typedef WizardDoneCallback = FutureOr<void> Function(Object? result);
lib/src/route.dart:    this.onDone,
lib/src/route.dart:  /// The callback invoked when the wizard is done.
lib/src/route.dart:  final WizardDoneCallback? onDone;
lib/src/scope.dart:  /// Returns `true` if the wizard is done.
lib/src/scope.dart:  bool get isDone => context.flow<List<WizardRouteSettings>>().completed;
lib/src/wizard.dart:  /// - [WizardScopeState.done]
lib/src/wizard.dart:  /// - [WizardScopeState.done]
test/wizard_router_test.dart:  Route? done;
test/wizard_router_test.dart:    done = null;
test/wizard_router_test.dart:    expect(observer.done, isNull);
test/wizard_router_test.dart:    expect(observer.done, isNull);
test/wizard_router_test.dart:    expect(observer.done, isNull);
test/wizard_router_test.dart:    expect(observer.done, isNull);

[jpnurmi]

LGTM 👍