Cleaning up some usings.

Ix.NET/Source/System.Linq.Async.Tests/System/Linq/Operators/GroupJoin.cs

@@ -5,8 +5,6 @@
 using System;
 using System.Collections.Generic;
 using System.Linq;
-using System.Threading;
-using System.Threading.Tasks;
 using Xunit;
 
 namespace Tests