Remove irrelevant api.Document.popupNode feature (#11722)
Remove irrelevant api.Document.popupNode feature (#11722)

This PR removes the irrelevant `popupNode` member of the `Document` API. The documentation for XUL documents has been removed from MDN, creating a broken link, and many other XUL features have been removed back in Firefox 64.
chrisdavidmills/browser-compat-data,chrisdavidmills/browser-compat-data,mdn/browser-compat-data,mdn/browser-compat-data