Remove feature flag to backfill partitioned table

What does this MR do?

Remove the backfill_partitioned_audit_events feature flag, which was default_enabled: true. The flag was added to provide an escape hatch for processing the audit_events partitioning background migration, in case of unforeseen performance impact on the production database.

Now that the migration has completed, it is no longer needed (and likely a new feature flag could be added for the next partitioning attempt).

Related issue: #251080 (closed)

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

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• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team