Detach grid hack on logical tree detachment.

Doing it on visual tree detachment caused the menu to bug out when showing a submenu more than once.

src/Avalonia.Controls/MenuItem.cs

@@ -325,6 +325,9 @@ namespace Avalonia.Controls
             {
                 Command.CanExecuteChanged -= CanExecuteChanged;
             }
+
+            _gridHack?.Dispose();
+            _gridHack = null;
         }
 
         protected override void OnAttachedToVisualTree(VisualTreeAttachmentEventArgs e)
@@ -350,13 +353,6 @@ namespace Avalonia.Controls
             }
         }
 
-        protected override void OnDetachedFromVisualTree(VisualTreeAttachmentEventArgs e)
-        {
-            base.OnDetachedFromVisualTree(e);
-            _gridHack.Dispose();
-            _gridHack = null;
-        }
-
         /// <summary>
         /// Called when the <see cref="MenuItem"/> is clicked.
         /// </summary>