Fix bookmarks writing in PDF.
Fix bookmarks writing in PDF.

The new prepare_metadata() already adds bookmark_root_id to
all values, no need to add it again in write_pdf_metadata()

andrewleech/WeasyPrint,prepare/TestWeasyPrint,marclaporte/WeasyPrint,Kozea/WeasyPrint,jasco/WeasyPrint,marclaporte/WeasyPrint,andrewleech/WeasyPrint,jasco/WeasyPrint,prepare/TestWeasyPrint,Kozea/WeasyPrint