Remove activities from schedule when events/rounds removed

[kr-matthews]

This won't help the existing competitions which have 'invalid' activities. I looked into that a bit but I'm not sure how/if that's causing the schedule view to crash, and I won't have time to look into in the near future. And if that's indeed causing the crash then we should fix the UI because other tools might remove events without removing them from the schedule (or alternatively, add more validation to the WCIF when saving it? I don't know much about that though).