Critical changes for developers & admins for Nextcloud 17

[rullzer]

Removed from public namespace:

• OCP\App::checkAppEnabled: Remove deprecated function checkAppEnabled #15781
• OCP\Security\StringUtils: Remove deprecatred StringUtils class #15779
• OCP\Util::callCheck: Remove deprecated function callCheck #15780

[ChristophWurst]

Dup of #15339 😉