Drop Feature Flag cluster_agents_limit_tokens_created

What does this MR do and why?

Drop the cluster_agents_limit_tokens_created Feature Flag.

This Feature Flag is for the logic to limit the creation of agent tokens to 2. With this FF dropped, the API to create an agent token will always return a 400 / error message if the agent already has 2 tokens.

MR introducing the change (including documentation): !120825 (merged). MR enabling the Feature Flag by default: !122848 (merged)

Feature Flag Issue: #412399 (closed)

Screenshots or screen recordings

N/A

How to set up and validate locally

N/A

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.

Related to #412399 (closed)