Corner case cancelling full booking - misleading description of behaviour

[pferreir]

Describe the bug
When cancelling a full booking which has started already, Indico indicates that it's going to remove all the occurrences, but then removes all occurrences minus one.

To Reproduce
Steps to reproduce the behavior:

1. Create a booking which spans over 8 days. Make it so that it starts exactly today;
2. Cancel the booking;
3. Indico will ask you if you want to delete the 8 occurrences - do so;

Expected behavior
All occurrences should go away (or Indico should let me know that one of the occurrences will stay nonetheless).

Screenshots

Additional context
This is was tested on Burotel. If it doesn't happen in vanilla Indico, then we should move it to the relevant repo.

[plourenco]

I tried to reproduce this in vanilla with 8 occurrences but all of them were cancelled.