Remove deprecated Agent token deletion mutation

What does this MR do and why?

This MR removes the deprecated clusterAgentTokenDelete GraphQL mutation.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Review progress:

• backend developer review
• backend maintainer review
• documentation review

Related to #349756 (closed)