Clean up ai_build_reader_for_chat feature flag

What does this MR do and why?

The ai_build_reader_for_chat has been rolled out so we are removing the flag.

It has to be merged only after its AI gateway counterpart: gitlab-org/modelops/applied-ml/code-suggestions/ai-assist!1617 (merged)

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

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Before	After

How to set up and validate locally

1. Check out branch 483189-clean-up-ai_build_reader_for_chat in AI Gateway.
2. Run GDK and AI gateway.
3. Visit a pipeline job and ask Duo Chat a question about the job: "What is this pipeline job about?"
4. You should receive a summary of the job.

Related to #483189 (closed)