Remove highlight_js feature flag

What does this MR do and why?

Removes thehighlight_js feature flag and related references.

Screenshots or screen recordings

No visual changes are expected as the feature flag is already enabled by default.

How to set up and validate locally

Open a source file via the repository files browser.

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 #366712 (closed)