Remove ai_events_backfill_to_ch feature flag

Pavel Shutsinrequested to merge
534895-cleanup-feature-flag into master

What does this MR do and why?

Removes ai_events_backfill_to_ch feature flag. It was introduced as part of https://gitlab.com/groups/gitlab-org/-/epics/17077+

The derisk feature flag was enabled everywhere for a while.

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Related to #534895

Edited by Pavel Shutsin

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