If they were one time timers they are already gone. If they were set to repeat then go to the right side, timers button and just delete them. I don't believe you can delete the history itself. They will self delete when approx a week old. My history of timers goes from last Saturday from this Saturday.
And they aren't really timers once they have become "old deleted/completed" events. They are a history of timers whether completed or skipped.