Fix drop_ai_* tables migrations

What does this MR do and why?

Fixes failing DB migrations

This is the follow-up after removal of the models Ai::DuoChatEvent, Ai::CodeSuggestionEvent and Ai::TroubleshootJobEvent

1. How the data could be recovered in the event of an incident.
   1. No need to recover, all data is already replicated in ai_usage_events and these tables are not in use anymore
2. The approximate number of records being affected.
   1. Not relevant
3. A description of the user experience impact.
   1. No impact

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Related to #605469 (closed)