Stop using the legacy cron function

[mdjnelson]

This will break when https://tracker.moodle.org/browse/MDL-61165 gets integrated.

Processing cron function for tool_objectfs...
++ Use of legacy cron is deprecated (tool_objectfs_cron). Please use scheduled tasks. ++
* line 3480 of /lib/deprecatedlib.php: call to debugging()
* line 172 of /lib/classes/task/legacy_plugin_cron_task.php: call to cron_execute_plugin_type()
* line 249 of /lib/cronlib.php: call to core\task\legacy_plugin_cron_task->execute()
* line 120 of /lib/cronlib.php: call to cron_run_inner_scheduled_task()
* line 73 of /lib/cronlib.php: call to cron_run_scheduled_tasks()
* line 178 of /admin/cli/cron.php: call to cron_run()
... started 20:48:39. Current memory use 89.9MB.