Clean up selectors in framework/header.scss

What does this MR do?

Removes .navbar-gitlab.navbar-gitlab selector and as a result integrates two blocks of .navbar-gitlab.container-fluid into a single block.

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

No.

Why was this MR needed?

The relevant code was introduced in !14573 (merged), and so to keep SCSS cleaned up.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• [n/a] Documentation created/updated
• [n/a] API support added
• [n/a] Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • [n/a] Has been reviewed by Backend
  • [n/a] Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• [n/a] Internationalization required/considered
• [n/a] End-to-end tests pass (package-qa manual pipeline job)

What are the relevant issue numbers?

Closes #44383 (closed)