Remove cluster_name from expel args

Summary

• node_expel.ms: remove cluster_name from picodata expel

• cli.md: remove cluster_name from picodata expel

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• Close nothing

• Cherry-pick to: 25.1, 24.6

• Staging:

  • https://docs.binary.picodata.io/picodata/dixitix/expel_without_cluster_name/

• See also picodata#1120 (closed)