Revert commit 9a41659 (PR 5005) for breaking the loading of certain PDF files in the Firefox addon when range requests are active

After https://github.com/mozilla/pdf.js/commit/9a41659ae7eddb92eaa09626db84b672947a6a43 (i.e. the first commit of PR #5005), the first page of http://purasilkthailand.com/static/doc/thai-tc.pdf no longer renders. This is *only* reproducible in the addon when range requests are active. To reproduce this, make sure that the file is loaded from the server by doing a "skip"-cache reload (e.g. <kbd>Ctrl</kbd> + <kbd>F5</kbd>).

What's somewhat unfortunate with this regression, is that not only has this already landed on Mozilla Central (see [bug 1042307](https://bugzilla.mozilla.org/show_bug.cgi?id=1042307)); but it has also been uplifted to the Aurora channel.

/cc @fkaelberer