Nomad inherited protocol version numbering configuration from Consul and
Serf, but unlike those projects Nomad has never used it. Nomad's
`protocol_version` has always been `1`.

While the code is effectively unused and therefore poses no runtime
risks to leave, I felt like removing it was best because:

1. Nomad's RPC subsystem has been able to evolve extensively without
   needing to increment the version number.
2. Nomad's HTTP API has evolved extensively without increment
   `API{Major,Minor}Version`. If we want to version the HTTP API in the
   future, I doubt this is the mechanism we would choose.
3. The presence of the `server.protocol_version` configuration
   parameter is confusing since `server.raft_protocol` *is* an important
   parameter for operators to consider. Even more confusing is that
   there is a distinct Serf protocol version which is included in `nomad
   server members` output under the heading `Protocol`. `raft_protocol`
   is the...