[feature/system-cron] Fixes to make cron actually run.

PHPBB3-9596

phpBB/includes/cron/cron_task_base.php

@@ -34,11 +34,6 @@ if (!class_exists('cron_task'))
 */
 abstract class cron_task_base implements cron_task
 {
-	/**
-	* Runs this cron task.
-	*/
-	abstract public function run();
-
 	/**
 	* Returns whether this cron task can run, given current board configuration.
 	*