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