feat: removeRoute now call didComplete (#157725)

**(edited 4 december) PR EDITED TO FIT LATEST COMMENTS**

Developper may want to remove a route in the background while an other
one is displayed.
To do so, developer can use removeRoute().

RemoveRoute() didn't resolve futures created by Navigator.push and so
code which await for Navigator.push will never end.

By calling complete() inside removeRoute, futures are well resolved. In
addition that also allow to pass some result through removeRoute.

Ex: 

```dart
Navigator.of(context).removeRoute(route, "test");
Navigator.of(context).removeRoute(route, object);
```

This PR aim to fix this issue :
https://github.com/flutter/flutter/issues/157505

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