Remove unused styles from header.scss

What does this MR do and why?

Remove unused styles from header.scss

• Move breadcrumbs and top-bar styles to isolated .scss files
• Remove .top-nav-menu-item styles and class usage

Screenshots or screen recordings

CleanShot_2023-12-18_at_16.24.40

How to set up and validate locally

1. Go to group e.g. http://gdk.test:3000/flightjs/
2. Open "Create" menu in sidebar
3. Press "Invite members" option, should have :hover/:focus/:active styles from GlDisclosureDropdownItem

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 #434154 (closed)