Kill support for legacy scrooge.

This change punts on cleaning up the ScroogeGen task
which now can be simplified a good deal.  The intent is
just to remove the possibility of folks binding to this capability
going forward.

Testing Done:
$ PANTS_DEV=1 ./pants goal test {src,tests}/{java,scala}::
$ PANTS_DEV=1 ./pants goal test tests/python/pants_test/:all
$ PANTS_DEV=1 ./pants goal test tests/python/pants_test/:integration

Bugs closed: 288, 289

Reviewed at https://rbcommons.com/s/twitter/r/603/