Remove support for the deleted Databinding API

Related to #491. There are several locations where WindowBuilder:

a) Makes several case-distinctions between the old and new Observable API
b) Creates a bi-directional mapping between methods of the new API to the old API

Keeping the former is a real pain in terms of maintainability. Latter falls apart because both versions are not 100% compatible.
Example: `SWTObservables.observeSelection()` can be any of `WidgetProperties.widgetSelection()`, `WidgetProperties.dateTimeSelection()`, ...
And this problem will only increase, the more features are added to the new API.

Given that the old API is gone for good, I don't think it makes any sense for WindowBuilder to keep referencing it internally or even generate code for it.