Do not persist events any longer to database

[nilmerg]

The event list has already lost its significance in version 0.2.0, as it now only displays events that sources have already assigned to specific event rules. The history of an incident also contains this information. There, it is also possible to see whether and which event rule processed the event.

While event rule matching will return with #390, the remaining reasons still apply and there's no real need to persist events any longer. The event list will therefore be removed in the UI without replacement. This also applies to the tables event and incident_event in the database.

Events will only be processed once and are then thrown away. There should be a debug log of received events and how they were handled, though. (If there isn't one yet, didn't check)