removeuseMaterial3: true, in from the template (#160525)

`useMaterial3` was `true` by default for some versions, so I think it
can be removed from the template.

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packages/flutter_tools/templates/app/lib/main.dart.tmpl

@@ -63,7 +63,6 @@ class MyApp extends StatelessWidget {
         // This works for code too, not just values: Most code changes can be
         // tested with just a hot reload.
         colorScheme: ColorScheme.fromSeed(seedColor: Colors.deepPurple),
-        useMaterial3: true,
       ),
       home: const MyHomePage(title: 'Flutter Demo Home Page'),
     );