Deprecate wheel tilt as separate axis source
This has never been supported through the stack. No device ever had the required MOUSE_WHEEL_TILT_VERTICAL/HORIZONTAL udev property set, so libinput never set the right axis source. Neither weston nor mutter added the code for it. Even if we added wheel tilt for devices now, it would break those devices. And the benefit we get from having those separate is miniscule at best.
So let's do the long-term thing and just deprecate this axis source.
The wheel tilt mouse test device remains in the test suite, with the udev properties set just to verify that we do indeed ignore those now.