Trim build.gradle to remove unneeded Avro dependencies

[ybyzek]

https://developer.confluent.io/tutorials/creating-first-apache-kafka-producer-application/confluent.html https://developer.confluent.io/tutorials/creating-first-apache-kafka-consumer-application/confluent.html

These tutorials don't use Avro (they produce/consume String keys and values), so the build.gradle file can be simplified