refactor: Remove scrollbars

They are now added automatically in Flutter 2.2

lib/pages/key_verification_dialog.dart

@@ -37,7 +37,6 @@ class KeyVerificationDialog extends StatefulWidget {
 
 class _KeyVerificationPageState extends State<KeyVerificationDialog> {
   void Function() originalOnUpdate;
-  final _scrollController = ScrollController();
 
   @override
   void initState() {
@@ -63,7 +62,6 @@ class _KeyVerificationPageState extends State<KeyVerificationDialog> {
         .contains(widget.request.state)) {
       widget.request.cancel('m.user');
     }
-    _scrollController.dispose();
     super.dispose();
   }
 
@@ -341,12 +339,8 @@ class _KeyVerificationPageState extends State<KeyVerificationDialog> {
       ],
     );
     final title = Text(L10n.of(context).verifyTitle);
-    final content = Scrollbar(
+    final content = SingleChildScrollView(
-      isAlwaysShown: true,
-      controller: _scrollController,
-      child: SingleChildScrollView(
       scrollDirection: Axis.vertical,
-        controller: _scrollController,
       child: Column(
         mainAxisSize: MainAxisSize.min,
         children: [
@@ -359,7 +353,6 @@ class _KeyVerificationPageState extends State<KeyVerificationDialog> {
           if (bottom != null) bottom,
         ],
       ),
-      ),
     );
     if (PlatformInfos.isCupertinoStyle) {
       return CupertinoAlertDialog(