Clean up importer_user_mapping flag

What does this MR do and why?

Clean up importer_user_mapping flag

This MR removes the importer_user_mapping feature flag. It leaves importer-specific flags in place. The overall effect is that general UCM behaviour should be enabled globally, but individual importers will still depend on their flags being enabled.

Changelog: other

References

#508944 (closed)

How to set up and validate locally

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.