Remove unused AgentMeta

This is a follow up for Stop registering agents in GetConfiguration (!1430 - merged). We don't need this field, so we can just remove it.

In practice this means new agent versions will not register when connecting to a kas instance that is older than %16.5, where we introduced a dedicated RPC (Refactor agent registration (#100 - closed)). Such configuration is not supported by our compatibility promise and we are braking compatibility in the above MR anyway.

Relates #493 (closed).