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();