Remove unused dependency on TraceSource

- TraceSource is RID specific (see https://github.com/dotnet/corefx/issues/7480) and it
causes 2 dlls to end up in the output of every ASP.NET application. We don't even
use it anymore.

src/Microsoft.AspNetCore.Server.Kestrel/project.json

@@ -36,8 +36,6 @@
       "dependencies": {
         "System.Collections": "4.0.11-*",
         "System.Diagnostics.Debug": "4.0.11-*",
-        "System.Diagnostics.TraceSource": "4.0.0-*",
-        "System.Diagnostics.Tracing": "4.1.0-*",
         "System.Globalization": "4.0.11-*",
         "System.IO": "4.1.0-*",
         "System.Linq": "4.1.0-*",