This reverts commit 10004c8.
Qt 4.8 doesn't support workers well enough to work with PDF.js. Specifically, ArrayBuffers don't seem to transfer well when sent to a worker. Ideally, PDF.js recognizes this empirically and disables workers itself (see api.js, WorkerTransport constructor). However, it doesn't realize this quickly enough for the first PDF load to succeed. By preemptively disabling workers, we make this problem go away. In the future if QtWebkit gets better worker support then we should take this out.