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 have an upstream version so we should do it here.
My only remaining suggestion is using
```
metadata.filename = new URL(documentURI).pathname.split('/').pop()
```
instead of
```
a = document.createElement 'a'
a.href = documentURI
metadata.filename = a.pathname.split('/').pop()
```