Remove CMSMenu controller routing rule when the controller is removed.

[sabina-talipova]

Description

When CMSMenu::add_controller() is called a director rule is added, but it won't be removed when the controller is removed. We should implement a way to remove the routing rule when the controller is removed.

See:

silverstripe-admin/code/CMSMenu.php

Line 73 in 0f18b3d

* @todo A director rule is added when a controller link is added, but it won't be removed