Cleanups to recent patch to pants goal depmap

Incorporated feedback from Patrick after submission of https://rbcommons.com/s/twitter/r/661/

Remove is_delegate attribute from AbstractTarget
Remove stray debug print message in a test

Testing Done:
ci passed at https://travis-ci.org/pantsbuild/pants/builds/30079790

Also ran pants goal depmap by hand on a dependencies() target in our repo

Bugs closed: 359

Reviewed at https://rbcommons.com/s/twitter/r/692/