Remove no-longer used inversion list function

Remove no-longer used inversion list function

The function _invlist_invert_prop() is hereby removed.  The recent
changes to allow \p{} to match above-Unicode means that no special
handling of properties need be done when inverting.

This function was accessible to XS code that cheated by using #defines
to pretend it was something it wasn't, but it also has been marked
as subject to change since its inception, and never appeared in any
documentation.