Clean Docker images

[delabassee]

Some images aren't used anymore, they should be removed.

• Java images are now in the FDK -> remove https://github.com/fnproject/dockers/tree/master/java
• Kotlin now rely on the Java images -> remove https://github.com/fnproject/dockers/tree/master/kotlin

Other unsupported images (ex. Erlang, Leiningen ?) should probably be tagged as experimental.