Move ONNX conversion to transformers

[Timoeller]

Currently FARM onnx conversion does not support all model types.

More specifically FARM does not adjust the naming and amount of input tensors to a model when converting to onnx. Implementing this for each model type can be cumbersome.

Transformers ONNX conversion

Transformers now has a onnx conversion functionality for their pipeline models (example notebook here). I tested the conversion for bert, roberta and xlm-roberta with success.
So lets focus more on FARM <> transformers interoperability like in this FARM example and refer people to their conversion script(s).

We will therefore soon deprecate FARMs ONNX conversion functionality