Resolve "The header navbar line separator is missing"

What does this MR do?

Bought back the line separator to the left of the 'Admin area' wrench icon that had vanished

Are there points in the code the reviewer needs to double check?

Probably app/assets/stylesheets/framework/header.scss:270, I removed a line that might impact other views. The line was probably added to set the search field to align to the center.

Why was this MR needed?

To fix #48634 (closed)

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Conform by the code review guidelines
  • Has been reviewed by a UX Designer
  • Has been reviewed by a Frontend maintainer
  • Has been reviewed by a Backend maintainer
  • Has been reviewed by a Database specialist
• Conform by the merge request performance guides
• Conform by the style guides
• If you have multiple commits, please combine them into a few logically organized commits by squashing them
• Internationalization required/considered
• End-to-end tests pass (package-and-qa manual pipeline job)

What are the relevant issue numbers?

Closes #48634 (closed)