Cannot remove the parent of a page

[tudoramariei]

Once a page has a parent, the field can never be set to null again.

Expected:

1. Given a page with parent X
2. Open the edit screen for that page
3. Set the parent to null/nothing/empty
4. Save the page

Actual:

1. Given a page with parent X
2. Open the edit screen for that page
3. There is no option to remove/set the parent to null