dialog test [nfc]: Remove an unused import

Looks like this slipped through CI, introduced recently in
8d3c128.

Author: chrisbobbe

test/widgets/dialog_test.dart

@@ -7,7 +7,6 @@ import 'package:url_launcher/url_launcher.dart';
 import 'package:zulip/model/settings.dart';
 import 'package:zulip/widgets/app.dart';
 import 'package:zulip/widgets/dialog.dart';
-import 'package:zulip/widgets/store.dart';
 
 import '../model/binding.dart';
 import 'dialog_checks.dart';