Fix build

Author: JamesNK

src/Servers/Kestrel/Core/src/Internal/Http2/Http2FrameWriter.cs

@@ -15,7 +15,6 @@
 using Microsoft.AspNetCore.Http;
 using Microsoft.AspNetCore.Server.Kestrel.Core.Internal.Http;
 using Microsoft.AspNetCore.Server.Kestrel.Core.Internal.Http2.FlowControl;
-using Microsoft.AspNetCore.Server.Kestrel.Core.Internal.Http2.HPack;
 using Microsoft.AspNetCore.Server.Kestrel.Core.Internal.Infrastructure;
 using Microsoft.AspNetCore.Server.Kestrel.Core.Internal.Infrastructure.PipeWriterHelpers;