incident: drop special handling of ACK events

[yhabteab]

The ACK event will still be processed just like before, but the special handling of the ACK event promoting the event author to incident manager is now completely removed as requested in the referenced issue. The event type TypeAcknowledgementSet and TypeAcknowledgementCleared are still not removed from Icinga Go Library, but since we're going to drop the type field from the event request with #407 anyway, it's not a problem to keep them around for now and not break existing implementations of Icinga DB with this change.

resolves #410

[Copilot]

Pull request overview

Removes the special-case handling of acknowledgement events in incident processing, where the ACK author was previously auto-promoted to incident manager. Acknowledgement events are still processed by the surrounding pipeline; only the manager-promotion side effect (and its associated helper, sentinel error, and history entry) is dropped. This implements the decision in #410 that managers should only be designated via the UI.

Changes:

• Replaces the switch ev.Type with a simple if ev.Type == baseEv.TypeState block, removing the TypeAcknowledgementSet case.
• Deletes the processAcknowledgementEvent helper and the errSuperfluousAckEvent sentinel error.
• Removes the now-unused errors import.

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