remove list item tree lines

• check memory usage widget
• check other items
• EE branch (e.g. approvals)

What does this MR do?

remove treelines

Are there points in the code the reviewer needs to double check?

most likely needs https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/12349 as well

Why was this MR needed?

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• [ ] Documentation created/updated
• [ ] API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes #33982 (closed)