Revert to using confluent-hub when it is is reintegrated into ksqlDB server

Revert https://github.com/confluentinc/kafka-tutorials/blob/master/_includes/tutorials/count-messages/ksql/code/docker-compose.yml#L94

cc: @colinhicks just to put it on the radar for when ksqlDB is upgraded from 0.11.0 to the next version (and not implying ksqlDB team has to do this, but if they wanted to, it would be a nice validation of the next release).