MDL-59159 upgrade: remove unused functions/settings from upgradelib

MDL-59159 upgrade: remove unused functions/settings from upgradelib

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()