Simplify/remove/clean code

* Remove useless runtime check (we only support one)
* Remove unused args
* Remove a global constant

And ofc fix some warnings ;)

Emby.Server.Implementations/HttpServer/StreamWriter.cs

@@ -1,6 +1,5 @@
 using System;
 using System.Collections.Generic;
-using System.Globalization;
 using System.IO;
 using System.Threading;
 using System.Threading.Tasks;
@@ -14,8 +13,6 @@ namespace Emby.Server.Implementations.HttpServer
     /// </summary>
     public class StreamWriter : IAsyncStreamWriter, IHasHeaders
     {
-        private static readonly CultureInfo UsCulture = new CultureInfo("en-US");
-
         /// <summary>
         /// Gets or sets the source stream.
         /// </summary>