Remove references to a return type for updateMessage/deleteMessage

[SimonWoolf]

cf https://ably.atlassian.net/wiki/spaces/CHN/pages/4549541909/PSDR5+return+type+of+updateMessage+deleteMessage+publish
ably/specification#403

new publishResponse return type to come soon, but for now, remove the old thing before the official preview release

[coderabbitai]

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✨ Finishing touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment
• Commit unit tests in branch update-no-return

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