Cleanup usings.

src/Avalonia.Controls/ContextMenu.cs

@@ -1,7 +1,6 @@
 using System;
 using System.ComponentModel;
 using System.Linq;
-using System.Reactive.Linq;
 using Avalonia.Controls.Generators;
 using Avalonia.Controls.Platform;
 using Avalonia.Controls.Primitives;

src/Avalonia.Controls/MenuBase.cs

@@ -7,7 +7,6 @@ using System.Linq;
 using Avalonia.Controls.Generators;
 using Avalonia.Controls.Platform;
 using Avalonia.Controls.Primitives;
-using Avalonia.Controls.Templates;
 using Avalonia.Input;
 using Avalonia.Interactivity;
 using Avalonia.LogicalTree;

src/Avalonia.Controls/Primitives/Popup.cs

@@ -2,12 +2,10 @@
 // Licensed under the MIT license. See licence.md file in the project root for full license information.
 
 using System;
-using System.Diagnostics;
 using System.Linq;
 using Avalonia.Input;
 using Avalonia.Input.Raw;
 using Avalonia.Interactivity;
-using Avalonia.Layout;
 using Avalonia.LogicalTree;
 using Avalonia.Metadata;
 using Avalonia.VisualTree;