Remove dangerous explicit destroy

There's no need to explicitly destroy the ClutterText actor inside the
label; doing so is actually harmful, as it will break the normal
reference cycle between container and children.
As StLabel doesn't hold any extra reference to the ClutterText actor and
just uses clutter_actor_add_actor() to add it to itself, let the normal
container dispose cycle run to dispose of the reference.

https://bugzilla.gnome.org/show_bug.cgi?id=783483

src/st/st-label.c

@@ -180,12 +180,6 @@ st_label_dispose (GObject   *object)
 {
   StLabelPrivate *priv = ST_LABEL (object)->priv;
 
-  if (priv->label)
-    {
-      clutter_actor_destroy (priv->label);
-      priv->label = NULL;
-    }
-
   g_clear_pointer (&priv->text_shadow_pipeline, cogl_object_unref);
 
   G_OBJECT_CLASS (st_label_parent_class)->dispose (object);