StWidget: don't dispose the theme node when destroyed

Theme nodes are interned and shared with other widgets, so they cannot
be disposed, otherwise we blow useful resources, and in particular we
break the parent-child chain.

https://bugzilla.gnome.org/show_bug.cgi?id=689029

src/st/st-widget.c

@@ -316,7 +316,6 @@ st_widget_dispose (GObject *gobject)
 
   if (priv->theme_node)
     {
-      g_object_run_dispose (G_OBJECT (priv->theme_node));
       g_object_unref (priv->theme_node);
       priv->theme_node = NULL;
     }