MDL-51580 upgradelib: Delete stuff used by removed upgrade steps

MDL-51580 upgradelib: Delete stuff used by removed upgrade steps

This commits removes stuff from different upgradelib files, used
exclusively by the already deleted upgrade steps. Given such
exclusivity it was not needed to proceed with a 2-phase deprecation
as far as the functions were 100% internal to upgrade.

This is the list of deleted functions, all them docummented in their
corresponding upgrade.txt files:

- repository_picasa_admin_upgrade_notification();
- repository_googledocs_admin_upgrade_notification();
- repository_boxnet_admin_upgrade_notification();
- repository_alfresco_admin_security_key_notice();
- qtype_essay_convert_to_html();
- portfolio_picasa_admin_upgrade_notification();
- portfolio_googledocs_admin_upgrade_notification();
- portfolio_boxnet_admin_upgrade_notification();
- mod_book_migrate_moddata_dir_to_legacy();
- mod_book_migrate_all_areas();
- mod_book_migrate_area();
- mod_assignment_pending_upgrades_notification();
- upgrade_mysql_fix_unsigned_and_lob_columns();
- upgrade_course_completion_remove_duplicates();
- upgrade_save_orphaned_questions();
- upgrade_rename_old_backup_files_using_shortname();
- upgrade_mssql_nvarcharmax();
- upgrade_mssql_varbinarymax();
- upgrade_fix_missing_root_folders();
- upgrade_course_modules_sequences();
- upgrade_grade_item_fix_sortorder();
- upgrade_availability_item();