Remove ICreatesObservableForProperty checks from WiXUiBootstrapperTests

This reduces the failing tests from 23 to 18.

I'll have to admit I'm not absolutely sure what ICreatesObservableForProperty is around in the code for now, but given the summary in ReactiveUI/RegisterableInterfaces.cs:

    /// <summary>
    /// ICreatesObservableForProperty represents an object that knows how to
    /// create notifications for a given type of object. Implement this if you
    /// are porting RxUI to a new UI toolkit, or generally want to enable WhenAny
    /// for another type of object that can be observed in a unique way.
    /// </summary>

and that the tests in `WhenAnyTests` all pass, it looks like things are safe