Remove ShutdownHook from jar-tool to prevent memory leak when running under nailgun.

This fixes a memory leak using nailgun with jar-tool caused by adding a Shutdown hook.

Testing Done:
Inspected with jvisualvm to find memory leak

In pants.ini, crank down the heap size:

 while [ t ] ; do PANTS_DEV=1 ~/Src/pants/pants goal bundle <target> --bundle-deployjar ; done

ran for a while and heap did not grow, whereas before it would die after just a few iterations with a 64 MB heap.

Bugs closed: 293

Reviewed at https://rbcommons.com/s/twitter/r/428/