Remove all Ant dependencies from JRuby proper

[headius]

JRuby has shipped support for Ant tasks for running Ruby and Rake code for many years, but Ant is now basically dead. I believe it's time to move the last pieces of Ant support out of JRuby, rework our build to not use any Ant tasks, and say goodbye.

There's several reasons for this:

• It is largely unmaintained code. It still works fine in our build, but we have no idea if it is fully functional in anyone else's Ant build.
• Ant is barely maintained itself. There does not seem to be any interest in adding Java Module support to it (other than in java/javac tasks) so we can't depend on it as a module either (see Modularize all the things #6598).
• Our use of Ant tasks in the build script requires Ant to be present on the host system, adding another dependency to JRuby development.
• The tasks we use are largely to help launching Java, and we have other ways to do that.
• As a separate project/artifact it can live and die on its own.