[ticket/10875] method_exists check is not required, interface declares them

The changes to the cache drivers added an interface, which requires many
cache functions exist. For these, we can remove the method_exists() check

PHPBB3-10875

phpBB/includes/cron/task/core/tidy_cache.php

@@ -43,8 +43,7 @@ class phpbb_cron_task_core_tidy_cache extends phpbb_cron_task_base
 	*/
 	public function is_runnable()
 	{
-		global $cache;
-		return method_exists($cache->get_driver(), 'tidy');
+		return true;
 	}
 
 	/**