Remove gitlab_ui_wg specific Dangerfile

What does this MR do?

This MR removes a special Dangerfile exception for MR's with a CSS Cleanup label. This seems to add unnecessary complexity and inconsistency to our reviewer selection process.

Why remove this?

• This might have been necessary when gitlab-ui was in its early stage. But now it's just confusing.
• Also, it's coupled with a CSS Cleanup label which isn't used consistently to represent GitLab UI working group changes.
• Also, it has hard coded references to people regardless of availability.