Remove OpenNMT-py code

[ruohoruotsi]

• Since OpenNMT-py is now on PyPI, we don't need to keep a full fork of the src in src/onmt

• We do need scorers and other utilities (like code to prepare a model for release, stripping out optimizer info and keeping only model weights and biases)

TODO:
Refactor top level scripts and code in src to use a pip installed OpenNMT-py for training and evaluation, using only the custom scoring and utils source where necessary.