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.
>  I don't follow you.

[Here](https://github.com/hypothesis/annotator/blob/d676c85561865f74fedc41cd72ee167125c8b21b/src/plugin/pdf.coffee#L248), you are creating a browser anchor to get the filename.

I think that will fail to get what we need while running as a Chrome extension (because the browser URL points to viewer.html), and it's redundant anyway. User the already calculated document URI for this.