Fix(Core): Prevent deletion of actors during escalation.

[stonebuzz]

Checklist before requesting a review

Please delete options that are not relevant.

• I have performed a self-review of my code.
• I have added tests (when available) that prove my fix is effective or that my feature works.
• I have updated the CHANGELOG with a short functional description of the fix or new feature.
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Description

• It fixes GLPI Escalade plugin all requesters and observers associated with the ticket are removed. #458

Wirth solution prposed here #458 (comment)

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