Libretexts: remove sandbox, private and semi-private pages

[benoit74]

While #104 has solved the general issue of rendering table of content, it did not took into account the special cases of Sandboxes, private and semi-private which are handled in original JS

It was deemed not worth the effort for v1 since it is not (yet?) encountered in the wild (private and semi-private pages are usually already excluded by other means, and we did not found a book containing sandbox pages), and the information needed is hard to obtain. Plus anyway it just means the ToC will contain these "hidden" entries, not something really harmful.

Do we need to do it at some point?