cardano-testnet: don't specify delegated supply anymore. Use create-testnet-data default behavior

[smelc]

Description

This pops an earlier TODO and doesn't cause any runtime changes, because the behavior remains the same under the hood.

Checklist

• [X] Commit sequence broadly makes sense and commits have useful messages
• [X] Self-reviewed the diff

[smelc]

Thanks @carbolymer :pray: