set appbar height (PreferredSize) (hide appbar)

 Scaffold(

  appBar:  PreferredSize(
    preferredSize: const Size.fromHeight(0),

    child: AppBar(

      title: const Text('My App Title'),
      elevation: 0,
    ),
  ),
  body: Container(

  ),
// This trailing comma makes auto-formatting nicer for build methods.
);