Fix the issue that GroupBy may not call 'unsubscribe'

Is this `isUnsubscribed` check only possible under a race condition between an `onComplete` and `unsubscribe` that comes after the check a few lines above? Do we really need this check any longer now that the WIP_FOR_UNSUBSCRIBE_UPDATER exists?

Note the comment a few lines above that we would have invoked `unsubscribe` if `groups.isEmpty()` and `child.isUnsubscribed() == true`. Those are two of the three conditions that would get us here IF the `unsubscribe` happens after we're past those lines. Thus it would be a race wit TERMINATED_WITH_COMPLETED and `unsubscribe`. 

Under that circumstance do we need the `isUnsubscribed` check? It seems fine to emit the `onCompleted` in that case since we are truly completed. The `unsubscribe()` would propagate up from the registered `Action` on line 102 if I understand correctly since this method would have decremented already.