Remove unused using.

Ix.NET/Source/System.Linq.Async/System/Linq/Set.cs

@@ -7,8 +7,6 @@
 using System.Collections.Generic;
 using System.Diagnostics;
 using System.Diagnostics.CodeAnalysis;
-using System.Threading;
-using System.Threading.Tasks;
 
 namespace System.Linq
 {