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().
1 job for !209 with work/pointer-axis-source-simplification in 7 minutes and 12 seconds (queued for 22 minutes and 6 seconds)
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passed linux kf5-qt5 qt5.15 #97472

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