Remove no holding external titles restriction

It turns out there's a new_title scope for the newly created title.
I don't need the ugly hack any more.

Unfortunately, as a result, creating titles for non-king vassals would
make them independent afterwards, so I've had to add a restriction that
you can only create High Kings from Kings.

This isn't really a big loss, as previously elevating a Duke to a High
King would make all his vassals independent.