Activation fixes.

This PR fixes some cases where the ViewModel could be Activated/Deactivated multiple times.

Although this was mostly fixed https://github.com/reactiveui/ReactiveUI/pull/550, this simplifies the ViewModel activation code significantly.

## Cases simplified:

* If the ViewModel was already set before the view got activated, we would activate it. It would then get deactivated because of the StartWith on `activation.Item2` and then be activated again when activation.Item1 would fire. We only need the last case, i.e. activate when the activation observable fires, so remove the other two situations which are unnecessary.
* It looks like we do not need the deactivation observable here so do not use it.