For |make firefox/mozcentral| builds, add cleanupJSSource to remove duplicate file headers in extensions/firefox/content/PdfjsChromeUtils.jsm

While looking at the attached patch in https://bugzilla.mozilla.org/show_bug.cgi?id=1064496, I'm seeing that my https://github.com/mozilla/pdf.js/pull/5115#issuecomment-54976863 was never addressed. This leads to a file header being present in the middle of the code, which doesn't look too good, hence this patch.

I should have checked this while working on PR #5305, but it unfortunately slipped my mind. Sorry about that!