Remove usings

Author: YishaiGalatzer

src/Microsoft.AspNet.Mvc.Core/Formatters/JsonOutputFormatter.cs

@@ -1,13 +1,8 @@
 // Copyright (c) Microsoft Open Technologies, Inc. All rights reserved.
 // Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
 
-using System;
 using System.IO;
-using System.Linq;
-using System.Text;
-using System.Threading;
 using System.Threading.Tasks;
-using Microsoft.AspNet.Http;
 using Microsoft.AspNet.Mvc.HeaderValueAbstractions;
 using Newtonsoft.Json;