The event scheduler logged start-, stop-execution messages in the error log
on lowest log-warning level which would lead to very large log files in 
a short time.
    
This patch changes this behavior so that no start-/stop-messages are logged.
Log messages stating successful execution are also removed.


sql/event_scheduler.cc:
  * Removed logging for start-/stop-events.
  * Removed log-messages stating successful execution of events.