Use delete_all for deleting events

What does this MR do?

Because we don't have any destroy callbacks (or other logic triggered on event destroy), there is no reason for deleting events inefficiently one by one, instead we can use :delete_all.

Related to https://gitlab.com/gitlab-org/gitlab-ce/issues/66926

Screenshots

Does this MR meet the acceptance criteria?

Conformity

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• Documentation created/updated or follow-up review issue created
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Performance and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers

Security

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