Remove extra using

Author: Mackinnon Buck

src/Servers/Kestrel/Core/src/Internal/Certificates/CertificateConfigLoader.cs

@@ -1,7 +1,6 @@
 // Licensed to the .NET Foundation under one or more agreements.
 // The .NET Foundation licenses this file to you under the MIT license.
 
-using System.Diagnostics;
 using System.Diagnostics.CodeAnalysis;
 using System.Globalization;
 using System.Security.Cryptography;