[Feature flag] Cleanup `kubernetes_namespace_for_environment`

What does this MR do and why?

This MR removes the kubernetes_namespace_for_environment FF, which is already enabled by default.

Screenshots or screen recordings

No visual changes

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.

Review progress:

• backend maintainer review
• frontend developer review
• frontend maintainer review
• documentation review

Related to #417129 (closed)