made use of unused derived_key_results.dart file on crypto_util.dart

lib/models/derived_key_result.dart

@@ -1,10 +1,9 @@
-// @dart=2.9
-
 import 'dart:typed_data';
 
 class DerivedKeyResult {
   final Uint8List key;
-  final Uint8List salt;
+  final int memLimit;
+  final int opsLimit;
 
-  DerivedKeyResult(this.key, this.salt);
+  DerivedKeyResult(this.key, this.memLimit, this.opsLimit);
 }

lib/utils/crypto_util.dart

@@ -6,6 +6,7 @@ import 'dart:typed_data';
 import 'package:computer/computer.dart';
 import 'package:flutter_sodium/flutter_sodium.dart';
 import 'package:logging/logging.dart';
+import 'package:photos/models/derived_key_result.dart';
 import 'package:photos/models/encryption_result.dart';
 
 const int encryptionChunkSize = 4 * 1024 * 1024;
@@ -330,11 +331,3 @@ class CryptoUtil {
     );
   }
 }
-
-class DerivedKeyResult {
-  final Uint8List key;
-  final int memLimit;
-  final int opsLimit;
-
-  DerivedKeyResult(this.key, this.memLimit, this.opsLimit);
-}