Removing unused using.

AsyncRx.NET/System.Reactive.Async/System/Reactive/Linq/Operators/DefaultIfEmpty.cs

@@ -2,8 +2,6 @@
 // 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.Collections.Generic;
-
 namespace System.Reactive.Linq
 {
     partial class AsyncObservable