It was causing problems (Windows build fails) now that we enabled  -Wweak-vtables (and
probably before didn't work that much before, guessing that's why we had that if (doc)
in openFile)

This is the simplest solution, invokeMethod is not great but we already
use it, so it's not too terrible

The openDocument function was unused so remove it.

The other two solutions are:
 * Make KDocumentViewer be part of okularcore and then link the
   okularcore to the okular binary, not nice
 * Make another dynamic library that just contains the KDocumentViewer
   class, but i'd rather not add yet another library we have to install
   and take care of