Remove bundling of pdf.js sources from this extension

https://jira.xwiki.org/browse/PDFVIEWER-6

We have a way that doesn't involve including the pdf.js sources in our code -> webjars, see https://github.com/webjars/pdf-js and http://extensions.xwiki.org/xwiki/bin/view/Extension/WebJars+Integration (Vincent's comment: https://github.com/xwiki-contrib/macro-pdfviewer/commit/3cf49ed020444ad76f7f4d18008d5ec3d4e5e881#commitcomment-5815747)