Remove more dead code and make minor fixes suggested by mypy

This changeset includes the following:

• Remove VendServer.DoorClient, which appears to be unused (and in any case, depends on the old LATClient, which has been removed).
• Fix function signature mismatch in Idler interface method getItemInfo, which was missing self.
• Don't try to use rsp before being assigned when logging a card enrollment failure.
• Add a few type hints, that allow mypy to verify correct type usage.