remove unused import in mock_client_test.dart (#76)

diff --git a/test/mock_client_test.dart b/test/mock_client_test.dart
index a19adca..48e635f 100644
--- a/test/mock_client_test.dart
+++ b/test/mock_client_test.dart
@@ -6,7 +6,6 @@
 import 'dart:convert';
 
 import 'package:http/http.dart' as http;
-import 'package:http/src/utils.dart';
 import 'package:http/testing.dart';
 import 'package:unittest/unittest.dart';