remove usings.

samples/ControlCatalog.NetCore/Program.cs

@@ -7,13 +7,11 @@ using System.Threading.Tasks;
 using Avalonia;
 using Avalonia.Controls;
 using Avalonia.Controls.ApplicationLifetimes;
+using Avalonia.Dialogs;
 using Avalonia.Headless;
 using Avalonia.LogicalTree;
-using Avalonia.Skia;
 using Avalonia.ReactiveUI;
 using Avalonia.Threading;
-using Avalonia.Dialogs;
-using Avalonia.OpenGL;
 
 namespace ControlCatalog.NetCore
 {