FIX: Ensure subcategory list is hidden when not required (PR #14424)

When the loading spinner is removed (e.g. via the loading-slider component), the subcategory list view will persist, even when no longer required. This is because we were conditionally rendering the list into the header-list-container outlet. When the condition was false, we were doing nothing. Instead, we should use disconectOutlet to make sure the content is removed from the DOM.