Don't hide widgets in dispose()

This is leftover code from when widgets were hidden
by default, and was setting them back to their initial
state.

This is getting in the way now, as hiding the widget
updates the HIDDEN accessible state, which ends up
re-creating the at context that we've already disposed
of, leading to memory leaks.

gtk/gtkwidget.c

@@ -7040,8 +7040,6 @@ gtk_widget_dispose (GObject *object)
 
   if (priv->parent)
     gtk_widget_unparent (widget);
-  else if (_gtk_widget_get_visible (widget))
-    gtk_widget_hide (widget);
 
   while (priv->paintables)
     gtk_widget_paintable_set_widget (priv->paintables->data, NULL);