Cleanup BaseTransitionModel

[zsoltk]

We have an excessive amount of related methods, do we really need all of them?

abstract fun ModelState.destroyedElements(): Set<Element<InteractionTarget>>

abstract fun ModelState.removeDestroyedElements(): ModelState

abstract fun ModelState.removeDestroyedElement(element: Element<InteractionTarget>): ModelState

abstract fun ModelState.availableElements(): Set<Element<InteractionTarget>>

FYI @KovalevAndrey