[build] (new) New 'distclean' target to remove RAIN's jar files

Hello,

In RAIN, current build targets don't include the possibility the remove prebuilt jars.
Specifically, both `clean` and `cleanall` targets only remove the `'bin` directory.

So, I've added the `distclean` targets that, in addition to calling the `cleanall` target, removes the RAIN's jar files (i.e., the `rain.jar` file as well as the jar files inside the `workloads` directory).

If you find this useful, can you consider to merge this pull request?

Best,

Marco