Remove usage of useMaterial3: true from ThemeData (#2383)

Part of [#162818](flutter/flutter#162818)

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Author: AbdeMohlbi

firebase-get-to-know-flutter/steps/index.lab.md

@@ -568,7 +568,6 @@ class App extends StatelessWidget {
           Theme.of(context).textTheme,
         ),
         visualDensity: VisualDensity.adaptivePlatformDensity,
-        useMaterial3: true,
       ),
       routerConfig: _router, // new
     );