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.
> URL isn't supported in IE but we could certainly polyfil it.

According to [the docs](https://developer.mozilla.org/en-US/docs/Web/API/URL), it's supported starting with IE10.