Remove 'explain_code_chat' feature flag

What does this MR do and why?

Contributes to #500094 (closed)

Removes outdated feature flag

References

Please include cross links to any resources that are relevant to this MR. This will give reviewers and future readers helpful context to give an efficient review of the changes introduced.

• Remove the obsolete version of Explain Code fea... (#500094 - closed)