Hotfix for clusters from #2701

[aerorahul]

Description

Fixes an issue created from #2701 that added CLUSTERS to the gaea.yaml.

Type of change

• Bug fix (fixes something broken)

Change characteristics

• Is this a breaking change (a change in existing functionality)? NO
• Does this change require a documentation update? NO

How has this been tested?

@AntonMFernando-NOAA found this on Hercules when trying to setup an experiment from develop. @aerorahul tested the setup_xml.py before and after this hotfix and #2701

Checklist

• [ ] Any dependent changes have been merged and published
• [ ] My code follows the style guidelines of this project
• [ ] I have performed a self-review of my own code
• [ ] I have commented my code, particularly in hard-to-understand areas
• [ ] My changes generate no new warnings
• [ ] New and existing tests pass with my changes
• [ ] I have made corresponding changes to the documentation if necessary

[DavidBurrows-NCO]

@aerorahul Thank you for getting this corrected...Is the plan to keep this hotfix in place or update MACHINE_ID.yaml files?