Clean Up removes named destinations that nothing in the document refers
to. referenced_names counted the /D of GoTo, GoToR and GoToE actions
alike. The /D of a GoToR or GoToE action names a destination in another
document, so a local destination with the same name was kept although
nothing in the document used it.
Only a GoTo's /D now counts as a use. A /Dest key and a GoTo's /D are
read as before.
Test: links with a GoTo, a GoToR and a GoToE action name three
destinations; optimize() keeps the one the GoTo uses and removes the
other two.