Drop schema retriever API and kcbq-api module

It's unlikely that the schema retriever interface is still useful; several advances have been made in how table creation and schema updates are performed by the connector since the API was originally added that do the work that the interface was intended to.

To simplify the code base, reduce maintenance cost, and lower the cognitive burden to users, we should remove the pluggable schema retriever interface and instead use the schemas provided with the records themselves by the Kafka Connect runtime (which is also the current default behavior of the connector).