Remove unused #tree-holder

What does this MR do?

1. Using CSS IDs is harder to maintain, so we should remove it
2. Apparently we don't even style or really use #tree-holder

As a result, this MR removes all instances of #tree-holder

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

Shouldn't be

Why was this MR needed?

Reduce ~"technical debt"

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
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• 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 https://gitlab.com/gitlab-org/gitlab-ce/issues/36035