Remove incorrect localVue usage in ee/spec/frontend/related_items_tree

What does this MR do?

As described in &2360 (closed), cleans up our Jest tests from unnecessary localVue usage

Involved specs

• ee/spec/frontend/related_items_tree/components/related_items_tree_app_spec.js
• ee/spec/frontend/related_items_tree/components/related_items_tree_header_spec.js
• ee/spec/frontend/related_items_tree/components/tree_item_body_spec.js
• ee/spec/frontend/related_items_tree/components/tree_item_spec.js

Conformity

• Merge request performance guidelines
• Style guides

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.

Closes: #121601 (closed)