When creating an activity with a grade type of 'Point' then changing it to 'None', the grade item entry is still present in the database and may be considered while aggregating the grades.

This was fixed for the aggregation method Natural in MDL-47681, but we need to investigate how this bug affects the stable branches and other aggregation methods, especially Sum of grades.

Along with this, we might want to:

• upgrade the database to set the maxgrade to 0 on those items
• prevent the activities from setting a maxgrade != 0 for those
• prevent the user from doing so from the setup screen

Switching from type Point to None has to be considered, not just locking the fields.