All these functions were used only by deleted upgrade steps
so it's safe to proceed with straight deletion, considering
them internal. Deletion has been documented in corresponding
upgrade.txt files:
- mod_feedback_upgrade_delete_duplicate_values()
- mod_feedback_upgrade_courseid()
These have been kept because they continue being used:
- @ install: make_competence_scale()
- @ restore: upgrade_course_letter_boundary()