Don't cancel calendar entries when meeting room gets removed

[tidoust]

The code expects a meeting to be a tuple: a room, a day, and a slot. If one of the information is missing, the code considers that the issue is not scheduled. One practical consequence is that unassigning the room for a group when a calendar entry had already been created will delete or cancel the calendar entry.

We should probably rather keep the calendar entry intact while the situation is being sorted. Unassigning a room typically means that event organizers are looking for a new room. That should be a temporary measure. If they wanted to cancel the meeting, event organizers would rather have dropped the day and slot.