Remove Banner embedded property

What does this MR do and why?

Here is the issue -> gitlab-org/gitlab-services/design.gitlab.com#1751 (closed)

There are no embedded banners in our projects, so we can clean it up

How to set up and validate locally

You can check the Banner component on lookbook http://gdk.test:3000/rails/lookbook/inspect/pajamas/banner/default

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.

#430244 (closed)