[DISCO-3036] refactor: r2d2 job to use domain mapping file

[tiftran]

References

JIRA: DISCO-3036

Description

Since we have the mapping file in merino already, there's no need to have the job ask for a category mapping file. It also simplifies the logic as well.

PR Review Checklist

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• [ ] This PR conforms to the Contribution Guidelines
• [ ] The PR title starts with the JIRA issue reference, format example [DISCO-####], and has the same title (if applicable)
• [ ] [load test: (abort|skip|warn)] keywords are applied to the last commit message (if applicable)
• [ ] Documentation has been updated (if applicable)
• [ ] Functional and performance test coverage has been expanded and maintained (if applicable)