Update letsencrypt.inc.php as per the discussion at ispconfig/ispconfig3#5030. Already pr'ed and merged in master via ispconfig/ispconfig3!778. Perhaps can be merged with stable too?

Added a third parameter to $app->log() function in server part to turn off logging to database and use this option in mysql lib to not log any server side SQL errors to the database.

every cron execution. The logfile can be disabled by using -NoLog

instead we are now using dynamic purging of the logs.