24726 Remove Across GitLab from side navigation

## What does this MR do?

Removes Across GitLab header from side navigation

## Screenshots (if relevant)
Before:
![Screen_Shot_2016-12-01_at_11.09.12_AM](/uploads/7a8c32a1736a55e0baedf071c21e98db/Screen_Shot_2016-12-01_at_11.09.12_AM.png)

After:
![Screen_Shot_2016-12-01_at_11.09.02_AM](/uploads/4d095ec6e4e756abf0301678bb40d59b/Screen_Shot_2016-12-01_at_11.09.02_AM.png)

## Does this MR meet the acceptance criteria?

- [ ] [Changelog entry](https://docs.gitlab.com/ce/development/changelog.html) added
- [ ] [Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md)
- [ ] API support added
- Tests
  - [ ] Added for this feature/bug
  - [ ] All builds are passing
- [ ] Conform by the [merge request performance guides](http://docs.gitlab.com/ce/development/merge_request_performance_guidelines.html)
- [ ] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [ ] Branch has no merge conflicts with `master` (if it does - rebase it please)
- [ ] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

## What are the relevant issue numbers?


Closes #24726

See merge request !7883