Delete page children tab dropdown toggler has no accessible name

There is no accessible name for the dropdown toggle of the children tab on the `Delete` pages. This is very probably generated in delete.vm. Note that the exact same element in restore.vm should be fixed at the same time.

Here is one of the WCAG reports mentionning it:

Validation in the test method [navigateWithKeyboardShortcuts]
Check for [org.xwiki.test.ui.po.ConfirmationPage] at http://xwikiweb:8080/xwiki/bin/delete/NestedNavigationIT/navigateWithKeyboardShortcuts/WebHome.
Found [2] items

2: Links must have discernible text
Description: Ensures links have discernible text
Help URL: https://dequeuniversity.com/rules/axe/4.6/link-name?application=axeAPI
Help: Links must have discernible text
Impact: serious
Tags: cat.name-role-value, wcag2a, wcag412, wcag244, section508, section508.22.a, ACT

HTML element:
<a class="panel-collapse-carret" role="button" data-toggle="collapse" href="#panel-children" aria-expanded="false" aria-controls="panel-children"><span class="caret"></span></a>
Selector: [.panel-collapse-carret]
Fix all of the following:
Element is in tab order and does not have accessible text

Fix any of the following:
Element does not have text that is visible to screen readers
aria-label attribute does not exist or is empty
aria-labelledby attribute does not exist, references elements that do not exist or references elements that are empty
Element has no title attribute

Here we can see the involved element, note that it only appears when trying to delete a page with at least one child.