Some cleanup.

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

@@ -2,9 +2,7 @@
 // The .NET Foundation licenses this file to you under the Apache 2.0 License.
 // See the LICENSE file in the project root for more information. 
 
-using System;
 using System.Collections.Generic;
-using System.Linq;
 using System.Threading;
 using System.Threading.Tasks;
 
@@ -363,4 +361,4 @@ namespace System.Linq
             return null;
         }
     }
-}
+}