Get rid of PointerAxisSource::Unknown
The idea behind this event source was to have a way to send artificial axis events. However, in hindsight, it doesn't make sense. If there is an event, then there must be its origin. Artificial axis events can have a dummy source, e.g. wheel.
By getting rid of the unknown source, we bring the internal axis source enum closer to the corresponding wl_pointer.axis_source enum, which allows us to simplify PointerInterface::sendAxis().