Drop OC_SubAdmin and replace usages

apps/provisioning_api/appinfo/routes.php

apps/provisioning_api/lib/groups.php

apps/provisioning_api/lib/users.php

apps/provisioning_api/tests/groupstest.php

apps/provisioning_api/tests/userstest.php

lib/private/api.php

lib/private/app.php

lib/private/group/metadata.php

lib/private/json.php

lib/private/legacy/subadmin.php

lib/private/subadmin.php

lib/private/util.php

settings/ajax/changedisplayname.php

settings/ajax/geteveryonecount.php

settings/ajax/setquota.php

settings/ajax/togglegroups.php

settings/ajax/togglesubadmins.php

settings/application.php

settings/changepassword/controller.php

settings/controller/groupscontroller.php