Remove sd-export-usb persistent USB device attachment

[rmol]

Removes the configuration of a single USB device persistently attached to sd-export-usb, in preparation for automatic attachment of export devices. Until that happens, the Qubes Devices tool should be used to attach them before exporting.

Fixes #320.

Testing

• In your dom0 clone of securedrop-workstation, run make remove-sd-export to remove your current configuration.
• Check out this branch in your source repository.
• Back in dom0, run make clone to pull these changes.
• (Optional) Remove the "usb" attribute from your config.json.
• Run make sd-export to recreate the export domains.
• Start sd-export-usb with Qube Manager.
• Attach a disk export device to sd-export-usb with Qubes Devices.
• Export something from the client, or if you have a disk export archive handy, just export it with qvm-open-in-vm sd-export-usb "filename".
• The export should succeed.

[emkll]

Thanks @rmol this looks good, make sd-export and make remove-sd-export seem to work for me locally, I've been having some issues getting a clean install run to complete (for reasons unrelated to this PR). We may need to rebase this on #325 once it gets approved/merged in order to fully test this.

Yesterday, we've merged changes in securedrop-export that mean that the config.json file for the export should no longer be required. As a result, I think the following 2 items can also safely be removed:

• This entire folder may no longer be necessary
• The specification of the config file may also no longer be necessary
• I also expect at least one test to fail, with the changes introduced here

[emkll]

Ran through the test plan, works exactly as advertised, thanks @rmol ! make test also has all (41) tests passing locally.

For those who forget to make clean on the previous branch, they can run qvm-usb remove sd-export-usb in dom0.