rethink cassandra-launcher approach

[raboof]

We're using sbt-assembly in an unusual way for cassandra-launcher - I'm not sure sbt-assembly really promised to do what we ask it to do here.

This causes trouble trying to update of sbt-multi-jvm or sbt-assembly.

[raboof]

hmm, this still seems to be brittle, breaking the build when plugins change (e.g. #205)

[Roiocam]

After some digging out in #206, cassandra-launcher has no users to sbt-assembly, but cassandra-bundle is used.

I think the sbt-assembly usage was in the "box", but the launcherbuild was a little problem.

Based on document: https://www.scala-sbt.org/1.x/docs/ Classpaths.html# Unmanaged+vs+managed
, Maybe we don't need to manage uber jar in the build