Remove “unlabeled” label from events.label

[osteele]

There's lot of special-casing around the maintenance of “unlabeled” label for events that don't otherwise have a label.

Remove this.

It's not currently used in the UI. If it is restored, it should be handled differently, by special-casing just the GET /events search and response. The currently implementation probably does not handle the cases of editing event label lists correctly, and would need to be better tested if it were maintained. It would also need some design around whether it's okay to have an item in events.labels that isn't in the labels collection, and whether it's valid to explicitly add "unlabeled" to an item.