Remove local path

This is the correct fix instead of https://github.com/hypothesis/annotator/pull/83, because I was not aware (and my browser caching issues prevented me from the seeing it) that for pdfs we do not use the Document plugin at all, our pdf plugin creates the metadata here, and that is what we need to change to fix https://github.com/hypothesis/h/issues/1661 

Our pdf plugin does not an upstream version so we should do it here.
I think the right way to get the URI is using the `_getDocumentURI()` method.
It should have both the protocol at the start, and the filename at the end.