Forum does not remove grades when reset

When a course reset is done with the option to remove all ratings from the forum (or in anything where the gradebook for the forum is reset and items removed) the grades held in the module in forum_grades is not removed.

Subsequently when the Whole Forum Grading UI is displayed, and there is an existing user from before the reset still in the class, their previous grade is reinstated in the UI (even though the gradebook is empty for them).

1. Find a forum with grading enabled
2. Grade the a user's post in the forum.
3. Check that grade appears in the gradebook
4. Verify grade appears in forum_grades. (DB Access required)
5. Reset course, setting forum to remove all ratings
6. Access the forum again and re-grade the user
7. Verify the grade from (2) appears in the grading form
8. Verify the grade in the grade book is null.