Removing unused using declaration.

Author: Benjamin Grabkowitz

src/IdentityModel.AspNetCore.OAuth2Introspection/Infrastructure/AsyncLazy.cs

@@ -2,7 +2,6 @@
 // Licensed under the Apache License, Version 2.0. See LICENSE in the project root for license information.
 
 using System;
-using System.Threading;
 using System.Threading.Tasks;
 
 namespace IdentityModel.AspNetCore.OAuth2Introspection.Infrastructure