Remove the “Random question using a new category” section from the “Add a random question” dialogue

As a teacher, I can add a 'random question' to my quiz where the question is taken from a question category (optionally including sub categories) with optional filtering by tags.

• You will no longer be able to add a new question category at the same time as creating a random question (there is not enough room to display a preview of the search as well as this create new question category option).