[Heartbeat] host.name should not be set in heartbeat

libbeat still [sets](https://github.com/elastic/beats/blob/master/libbeat/publisher/processing/default.go#L80) `host.name` on all events even with the `add_host_metadata` processor removed. We should remove it, as heartbeat is an `observer` not a `host` in ECS parlance. There is [prior art](https://github.com/elastic/apm-server/blob/master/cmd/root.go#L83) here in APM.