Fix updating events for WikiPage::Meta conflicts

What does this MR do and why?

Follow up to !180695 (merged).

Fixes https://new-sentry.gitlab.net/organizations/gitlab/issues/1349759/?project=3

When deciding to move activity events from the duplicate WikiPage::Meta record to the other one by simply updating the target_id, we failed to account for a unique index on (target_type, target_id, fingerprint) for the events table.

The fingerprint (which is a wiki page SHA) is the same in a scenario when a new wiki page is created and two duplicate events are created, one for each Meta. They share the same fingerprint, and we violate a unique index when we try to both have the same target_id.

Instead of updating the records, we opted to let them be destroyed along with the duplicate Meta record.

References

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• Handle duplicate WikiPage::Meta record conflicts (!180695 - merged)
• Wiki comments fail to load if a page had a dupl... (#508805 - closed)

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