how to remove/stop system?

I want to implement Hud system that would add, update and remove all the UI elements. The thing is, I want this to work only on few game states. So I need to be able to start system in `on_start` and to stop them in `on_stop`.
I suppose, I can add system using [Builder.add_system](https://docs.rs/legion/0.4.0/legion/systems/struct.Builder.html#method.add_system) But how can I remove system?