Kill unused antlr deps in 3rdparty/BUILD.

Add a TODO in PythonAntlrBuilder to merge with
the AntlrGen task - right now PythonAntlrBuilder
does ~out of band ivy resolution of antlr tools.

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

Bugs closed: 322

Reviewed at https://rbcommons.com/s/twitter/r/642/