app-system: Remove get_tree

Make clients construct their own gmenu tree if they need it.

https://bugzilla.gnome.org/show_bug.cgi?id=698486

src/shell-app-system.c

@@ -390,17 +390,6 @@ on_apps_tree_changed_cb (GMenuTree *tree,
   g_signal_emit (self, signals[INSTALLED_CHANGED], 0);
 }
 
-/**
- * shell_app_system_get_tree:
- *
- * Return Value: (transfer none): The #GMenuTree for apps
- */
-GMenuTree *
-shell_app_system_get_tree (ShellAppSystem *self)
-{
-  return self->priv->apps_tree;
-}
-
 /**
  * shell_app_system_get_default:
  *