Students with mod/assign:viewgrades set to Prohibit can still view their grades

It's been found that users with Student role can view their grades even if mod/assign:viewgrades is set to Prohibit.

Steps to replicate the issue:

1. Create a course and a user.
2. Enrol the user to the course with Student role.
3. Create an assignment acitivity.
4. Login as student and submit a submission.
5. Login as teacher/admin and grade the submission.
6. Edit Student role and set mod/assign:viewgrades capability to Prohibit.
7. Login as student and access the activity.
8. Confirm, that Feedback section is visible and student can view their grade.