When attempting to delete courses, if the course has been changed with a custom grade letter setting the delete attempt will fail with the following errors:
 

Exception - Call to undefined method mysqli_native_moodle_database::delete_record()

More information about this error
 
Debug info:
Error code: generalexceptionmessage
Stack trace:

• line 1404 of /lib/gradelib.php: Error thrown
• line 4924 of /lib/moodlelib.php: call to remove_grade_letters()
• line 4840 of /lib/moodlelib.php: call to remove_course_contents()
• line 67 of /course/delete.php: call to delete_course()

Steps to reproduce:

1. Inside of a course, click on gradebook setup
2. Click on Letters
3. Click on Edit grade letters
4. Click on override site defaults
5. Click on save
6. Attempt to delete the course