[Feature flag] Remove `project_import_level_configuration` feature flag
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## Summary
We no longer need the `project_import_level_configuration` feature flag as we are taking a different direction and not adding a configurable setting. We should therefore remove it.
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## Owners
- Team: Group::Import
- Most appropriate slack channel to reach out to: `#g_manage_import`
- Best individual to reach out to: @carlad-gl
- PM: @m_frankiewicz
Tasks:
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- [ ] Create a merge request to remove `<project_import_level_configuration>` 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 included in the release package.
- [ ] `/chatops run release check <merge-request-url> <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 --staging --staging-ref --production`
- [ ] Close this issue.
issue