Remove persistent attachment to sd-export-usb VM

[eloquence]

In #259 we added support for exporting to USB drives which relies on a persistent attachment of a USB device to a disposable VM.

In practice we've encountered significant issues with this approach; notably, sys-usb will sometimes associate different device IDs with different storage devices attached to the same USB port (#307), and the device has to be plugged in at the time the VM starts.

We're therefore considering transitioning to an approach which relies on automatically attaching certain classes of devices (USB mass storage, printers) to the sd-export-usb VM. Even if that approach does not prove viable, the persistent attachment clearly isn't. Our fallback is to rely on the device manager in Qubes to handle the attachment manually.

For these reasons, we should remove the persistent attachment requirement from the code and configuration for the export VM.