Enable binary comp testing. Final build cleanup.

This will hopefully by my last changes to build.xml.
For my grand finale, reduced the line & file count to an all-time low.

Review by @gkossakowski

Nightly job running to double check the build changes don't break it:
  - https://scala-webapps.epfl.ch/jenkins/job/scala-checkin-manual/1206/console
  - https://scala-webapps.epfl.ch/jenkins/job/scala-checkin-manual-windows/40/console

Note: the next item on my TODO list is to build full scala releases nightly, to replace what I removed here (and then some).