Remove the feature_flag code_basic_search_files_by_regexp

What does this MR do and why?

This MR will cleanup the feature_flag code_basic_search_files_by_regexp

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

How to set up and validate locally

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 #389551 (closed)