MDL-20154 quiz: Remove unused random questions

When a random question is removed from a quiz, and it is not
in use elsewhere, it should be deleted.

Conflicts:
	mod/quiz/editlib.php
	mod/quiz/lib.php
MDL-20154 questions: question_delete_question should not check perms

By Moodle convention, library functions should not implement permission
checks, they are to be handled by the invoking code.