Forbidding delete right transforms undelete right in "un" right

To reproduce:

• create a page
• go to the edit -> access rights UI
• explicitely give delete right to a group or user
• go in the object editor, you'll see the XWikiRights object with allow and the "delete" right selected
• change the selection to give the "undelete" right. You may keep or not keep the delete right as well, you will still see it in the UI because of XWIKI-7148.
• go back to the access rights editor, and forbid the delete right (click the checkbox one more time)
• view the page with xwiki/bin/view/space/page?xpage=xml, you should find a rights object with a property

  <levels>un<levels>

  and no object that actually gives the undelete right