When we receive the 'objects-modified' signal, we get components that may represent main events. We also may receive components that have no recurrency because they were just changed from being recurrent to non-recurrent. We need to handle all these cases properly. If the event has no recurrency set, remove any potential old recurrence instance lingering around. If we're changing an event with recurrency, make sure the recurrency instances are in sync with the new recurrency. Fixes https://gitlab.gnome.org/GNOME/gnome-calendar/-/issues/881 (or so I hope.)