Eloy Lafuente authored Jun 14, 2017 and Dan Poltawski committed Jul 10, 2017

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()