Some code cosmetics.

Ix.NET/Source/System.Interactive.Async/Except.cs

@@ -4,7 +4,6 @@
 
 using System.Collections.Generic;
 using System.Diagnostics;
-using System.Threading;
 using System.Threading.Tasks;
 
 namespace System.Linq

Ix.NET/Source/System.Interactive.Async/Expand.cs

@@ -4,7 +4,6 @@
 
 using System.Collections.Generic;
 using System.Diagnostics;
-using System.Threading;
 using System.Threading.Tasks;
 
 namespace System.Linq