Don't ignore events from XI2 slave devices anymore

Don't ignore events from XI2 slave devices anymore

All the machinery needed to keep track of events from those
devices is already in place, so that's no longer required.

* src/xterm.c (x_get_scroll_valuator_delta):
(xi_reset_scroll_valuators_for_device_id):
(handle_one_xevent): Don't ignore XI devices that are not master
pointers or keyboards.