Remove qr/\p{_Case_Ignorable}/

Remove qr/\p{_Case_Ignorable}/

This property is no longer used in the core, nor in cpan, and is marked
as for core use only, not necessarily stable.  I have kept it around
because it was work to remove it, but now the revamping of the property
lookup scheme was causing failures with it, when compiling on early
Unicode releases.  That could be fixed with extra work, but simply
removing it also fixes the problem and avoids future maintenance
costs.