=act #3623 unsubscribe when actors terminate, instead of `isTerminated`

Moved to removing actors proactively when they are terminated instead of checking `isTerminated` during publish.

Assumed having an `init` method on the `EventBus` is fine, as it is mutable already anyway, as @patriknw points out in the task.

Related question: should this change be documented in the docs too? I figured that it's more of an internals change so maybe not?