Remove ICreatesObservableForProperty checks from WiXUiBootstrapperTests

[rzhw]

This reduces the failing tests from 23 to 18.

I'm not absolutely certain 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, as well as the use of WhenAny for only IReactiveNotifyPropertyChangeds i.e. being covered by WhenAnyShim, it looks like things are safe