Delete DropDown.cs

It wasn't supposed to live for so long.

src/Avalonia.Controls/DropDown.cs

@@ -1,28 +0,0 @@
-using System;
-using Avalonia.Logging;
-using Avalonia.Styling;
-
-namespace Avalonia.Controls
-{
-    [Obsolete("Use ComboBox")]
-    public class DropDown : ComboBox, IStyleable
-    {
-        public DropDown()
-        {
-            Logger.TryGet(LogEventLevel.Warning, LogArea.Control)?.Log(this, "DropDown is deprecated: Use ComboBox");
-        }
-
-        Type IStyleable.StyleKey => typeof(ComboBox);
-    }
-
-    [Obsolete("Use ComboBoxItem")]
-    public class DropDownItem : ComboBoxItem, IStyleable
-    {
-        public DropDownItem()
-        {
-            Logger.TryGet(LogEventLevel.Warning, LogArea.Control)?.Log(this, "DropDownItem is deprecated: Use ComboBoxItem");
-        }
-
-        Type IStyleable.StyleKey => typeof(ComboBoxItem);
-    }
-}