Remove unneccessary using directives.

src/Avalonia.Controls/Label.cs

@@ -1,12 +1,5 @@
-using System;
-using System.Collections.Generic;
-using System.Runtime.CompilerServices;
-using System.Text;
 using Avalonia.Automation.Peers;
 using Avalonia.Controls.Automation.Peers;
-using Avalonia.Controls.Primitives;
-using Avalonia.Controls.Templates;
-using Avalonia.Data;
 using Avalonia.Input;
 using Avalonia.Interactivity;