Removed unused using.

src/Avalonia.Controls/ItemsControl.cs

@@ -4,7 +4,6 @@
 using System.Collections;
 using System.Collections.Generic;
 using System.Collections.Specialized;
-using System.Diagnostics.CodeAnalysis;
 using System.Linq;
 using Avalonia.Collections;
 using Avalonia.Controls.Generators;