Remove ai_gateway_docs_search flag
What does this MR do and why?
Remove ai_gateway_docs_search flag. Only use AI Gateway to do doc search.
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.
How to set up and validate locally
Numbered steps to set up and validate the change are strongly suggested.
Related to #451215 (closed)