Remove Terraform duplication in `streaming/`

* See [this comment](https://github.com/GoogleCloudPlatform/kubernetes-engine-samples/pull/725#discussion_r1316150606).
* The Terraform inside `/streaming/kafka-strimzi/terraform` is mostly identical to the Terraform added by https://github.com/GoogleCloudPlatform/kubernetes-engine-samples/pull/725.
* We should minimize this duplication.
* This would involve:
* [ ] creating more generic (re-usable) Terraform (e.g., inside `/streaming/kafka/terraform`)
* [ ] updating the two tutorials (Strimzi and Confluent) to use the new generic Terraform (`/streaming/kafka/terraform`)
* [ ] removing unused Strimzi-specific and Confluent-sepecific Terraform from this repo