Removes EE differences for app/views/layouts/fullscreen.html.haml

What does this MR do?

Removes EE differences for app/views/layouts/fullscreen.html.haml

Closes https://gitlab.com/gitlab-org/gitlab-ee/issues/12550

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• [-] Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• Separation of EE specific content