migrated delete_account.dart to null safety

lib/models/delete_account.dart

@@ -1,13 +1,9 @@
-// @dart=2.9
-
-import 'package:flutter/foundation.dart';
-
 class DeleteChallengeResponse {
   final bool allowDelete;
   final String encryptedChallenge;
 
   DeleteChallengeResponse({
-    @required this.allowDelete,
-    this.encryptedChallenge,
+    required this.allowDelete,
+    required this.encryptedChallenge,
   });
 }