Logstash. Get fields by position number

Elastic Stack Logstash
3

I want to remove them completely.
I got an advice on stackoverflow to use filter like this:

grok {
     pattern => "%{TIMESTAMP_ISO8601:timestamp} %{WORD:app} %{WORD:server} %{GREEDYDATA:message}"
     overwrite => [ "message" ]
}

json {
    source => "message"
}

But now I'm getting error in log about "Trouble parsing json".