Remove any auditor functionality from server

[masomel]

Since the CONIKS auditor will be a separate software package, the CONIKS key server doesn't need to contain any functionality that also makes it an auditor (e.g. reply to observed STR requests). However, once auditors are implemented, the server may end up needing a mechanism for distributing its signed tree roots to auditors.

This also includes editing the protobufs.