Remove "fulltext index" from library selector

[Popolechien]

This selector is a) hard to understand and b) provides no visible information value to the end user ("what's a fulltext index? Can I get the same content with and without index? What am I losing and how do I know it?") and thus could be removed so as to de-clutter the library interface.

[kelson42]

Agree with this remark. Similar to kiwix/libkiwix#545