Remove the separate C* Docker image used for encryption

[whatyouhide]

C* supports "optional" encryption, that is, it supports clients that can connect both via TCP as well as TLS (on the same port). If we enable this in C*'s config, we can remove the cassandra_encrypted Docker Compose service from docker-compose.yml. Eventually, we might even want to re-run the whole test suite under TCP and TLS to test edge cases, but for now that won't be necessary.