[T5] Unused `n_positions` and `max_position_embeddings`.
The T5Config has the parameter `n_positions` set to 512 and `max_position_embeddigs` referring to `n_positions`. However, neither `max_position_embeddigs` nor `n_positions` is used in the `T5Model` and T5 is not limited to `max_position_embeddings`. *E.g.*:

```python
from transformers import T5Model
model = T5Model.from_pretrained("t5-small")

model.config.max_position_embeddings # shows 512

input_ids = torch.tensor([600 * [0]])  # input of size > 512

model(input_ids, decoder_input_ids=input_ids)  # works fine
```

 I think we should delete the parameter. 

@thomwolf - do you remember why we added `max_position_embeddigs` and `n_positions` to T5? The model does not seem to use these params and also should not be limited to 512 due to its relative position embeddings.