Remove backend core.
Review Request #3324 - Created Jan. 9, 2016 and submitted
The only thing remaining in it was the old builddict generation code (no longer used), and a test that didn't belong there anyway.
Jenkins CI passes: http://ec2-54-146-169-50.compute-1.amazonaws.com:8080/job/pants_ci_trigger/64/
Travis too: https://travis-ci.org/pantsbuild/pants/builds/101190468