[Feature flag] Cleanup importer_user_mapping

<!-- Title suggestion: [Feature flag] Cleanup <feature-flag-name> --> ## Summary This issue is to cleanup the `importer_user_mapping ` feature flag, after the feature flag has been enabled by default for an appropriate amount of time in production. <!-- Short description of what the feature is about and link to relevant other issues. Ensure to note if the feature will be removed completely or will be productized--> ## Owners - Team: ~"group::import" - Most appropriate slack channel to reach out to: `#g_import` - Best individual to reach out to: `@rodrigo.tomonari` - EM: `@adebayo_a` ## Stakeholders <!-- Are there any other stages or teams involved that need to be kept in the loop? - Name of a PM - The Support Team - The Delivery Team --> ## Expectations ### What might happen if this goes wrong? <!-- Any MRs that need to be rolled back? Communication that needs to happen? What are some things you can think of that could go wrong - data loss or broken pages? --> ### Cleaning up the feature flag <!-- The checklist here is to help stakeholders keep track of the feature flag status --> - [ ] Specify in the issue description if this feature will be removed completely or will be productized as part of the Feature Flag cleanup - [ ] Create a merge request to remove `<feature-flag-name>` feature flag. Ask for review and merge it. - [ ] Remove all references to the feature flag from the codebase. - [ ] Remove the YAML definitions for the feature from the repository. - [ ] Create [a changelog entry](https://docs.gitlab.com/ee/development/feature_flags/#changelog). - [ ] Ensure that the cleanup MR has been deployed to both production and canary. If the merge request was deployed before [the code cutoff](https://about.gitlab.com/handbook/engineering/releases/#self-managed-releases-1), the feature can be officially announced in a release blog post. - [ ] `/chatops run auto_deploy status <merge-commit-of-cleanup-mr>` - [ ] Close [the feature issue](ISSUE LINK) to indicate the feature will be released in the current milestone. - [ ] If not already done, clean up the feature flag from all environments by running these chatops command in `#production` channel: `/chatops run feature delete <feature-flag-name> --dev --pre --staging --staging-ref --production` - [ ] Close this rollout issue. <!-- Uncomment the appropriate type label /label ~"type::feature" ~"feature::addition" /label ~"type::maintenance" /label ~"type::bug" -->

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