Remove _nullBuffer

Was meant to be removed as part of #335

/cc @rynowak

src/Microsoft.AspNet.Server.Kestrel/Http/Frame.cs

@@ -32,7 +32,6 @@ public partial class Frame : FrameContext, IFrameControl
         private readonly object _onStartingSync = new Object();
         private readonly object _onCompletedSync = new Object();
         private readonly FrameRequestHeaders _requestHeaders = new FrameRequestHeaders();
-        private readonly byte[] _nullBuffer = new byte[4096];
         private readonly FrameResponseHeaders _responseHeaders = new FrameResponseHeaders();
 
         private List<KeyValuePair<Func<object, Task>, object>> _onStarting;