cli/demo: the \demo recommission command has become non-sensical since v20.2

[knz]

In v20.2 we have introduced the 2-step state transition decommissioning -> decommissioned.
once a node is in the "decommissioned" state the state is permanent -- i.e. it cannot be recommissioned.

Meanwhile, the \demo decommission command performs both state changes i.e. it has become unrecoverable since v20.2.

This means that \demo recommission simply cannot exist any more. It should be removed.

Epic: CRDB-28893

Jira issue: CRDB-30522