Remove wParentIndex from XFA_ELEMENTHIERARCHY.
Remove wParentIndex from XFA_ELEMENTHIERARCHY.

This value is never set and never read. Removing it fixes a series of warnings
when building on OSX as g_XFAElementPropertyIndex does not initialize the
value.

R=tsepez@chromium.org

Review URL: https://codereview.chromium.org/1744893002 .

DrAlexx/pdfium,DrAlexx/pdfium,DrAlexx/pdfium,DrAlexx/pdfium