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:
- upgrade_mimetypes()
- upgrade_fix_missing_root_folders_draft()
- upgrade_minmaxgrade() and upgrade_minmaxgradestepignored setting
- upgrade_course_tags()
- atto_equation_update_librarygroup4_setting()
- mod_lti_upgrade_custom_separator()
These have been kept because continue being used by restore:
- upgrade_group_members_only()
- upgrade_extra_credit_weightoverride()
- upgrade_calculated_grade_items()
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