Fix incorrect calls to non-existent delete_record() function

Test 1 (to be verified in 35_STABLE and master) Create a course. Visit the course. Click on 'Grades' in the course. Click on the tab 'Letters'. Click on 'Edit grade letters'. Check 'Override site defaults'. Save changes. Click on 'Edit grade letters'. Uncheck 'Override site defaults'. Save changes. Ensure you did not experience an exception. Test 2 (to be verified in 34_STABLE, 35_STABLE and master) Create a course. Add a lesson module. Click 'Add a cluster'. Visit the database table 'lesson_answers' using whatever tool you prefer. Duplicate the row that was added. Back on the Edit page of the lesson module in Moodle, click on the 'cog' icon on the " Actions " column Click 'Save page'. Ensure you did not experience an exception. Test 3 Everything else is covered by unit tests.