When a sub question category is move up into a parent category it is renamed and changed into a second top level category which is incorrect behaviour and breaks the categories.

Steps to reproduce:

1. Log in as admin
2. Create a course (c1)
3. Create a quiz activity (quiz1) in c1
4. View quiz1
5. Open the question bank from quiz1
   • edit cog > question bank
6. Click the "Categories" tab
7. Add a sub course category to c1
   • It's parent category should be set to "Default for c1" to create this as a sub category
8. Use the left arrow to move it up a category level from a sub category. It will be incorrectly renamed to "Default for c1" and the category page will no longer work (turning developer debugging on will show an error message).
9. Clicking the right arrow to demote it back to a sub category will create multiple sub categories in the other "Default for c1" category