Decide what to do with ProverStandalone

[David Aspinall <da+pgtrac@…>]

Should this code be retained or discarded? Not clear how it fits into future development plans, and already changes elsewhere in code have likely broken it. One possibility may be to move it to another plugin.

In the longer term, we've talked about adding an API to the Broker to manage running provers, so the code here would form a basis for that.