calendar: Don't allow event messages to be closed

The built-in calendar isn't meant to replace a full-fledged calendar app, which is why clearing event messages only hides the event in gnome-shell rather than deleting the actual event. This has turned out to not be overly useful and often confusing - it creates a discrepancy with visible events in apps, isn't revertible in a non-obscure fashion and non-obviously limited to the current date.

As we are considering moving events out of the message list and back to the calendar, it looks like a good time to remove that ability and keep notifications as the only removable messages.

Related: #262 (closed)