Use `delete_all` instead of `destroy` when deleting events
The following discussion from gitlab-ee!15649 should be addressed:
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@jprovaznik started a discussion: In this case we can't rely on DB constraint and we have to add
dependent: ...because it's a polymorphic association.Surprisingly other eventable resources use
dependent: :destroywhich I think is not necessary because we don't use any callbacks on Event model. So perhaps these should be changed todelete_allto in a follow-up.