Clean up environment_details_vue feature flag

What does this MR do and why?

This has been rolled out for a few milestones now, and no issues have been reported. Let's remove the flag!

Changelog: other

Screenshots or screen recordings

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

Before	After

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

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.

For #410159 (closed)