Random question type should be removed (because there are better ways to handle random questions)

The ability to select questions at random from the question bank is an important feature. However, the way it is currently implemented, with instances of a fake question type sort-of present in the question bank but hidden, causes no end of problems, particularly with backup and restore. There are better ways to store what is going on.

For example, in the quiz, the quiz_slots DB table has a questionid column that directly links to the question for non-random questions, and it has columns categoryid + tag + include subcategories, for randomly selected questions. That is the good way to handle this, and does not require qtype_random at all.

So, qtype_random should be removed (except, possibly, for a stub to support restore of old backups). And all the code simplification that follows from that should be made.