[T5] Bug correction & Refactor
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**!!!BUG DETECTION!!!** 

While integrating T5v1.1/mT5, a bug in T5 was detected. T5 actually never uses a `relative_position_bias` in the `EncDecSelfAttention` layer. Previously we used a bi-directional `relative_position_bias` in the `EncDecSelfAttention`, which is wrong IMO (see https://github.com/huggingface/transformers/issues/6285#issuecomment-702371111 for reference). An integration test against original T5 model was added to make sure removing `relative_posiiton_bias` is the correct behavior (in case @craffel reads this - maybe you could confirm :-)). 

Luckily, the bug did not significantly influence the results as can be seen by the very minor changes in the slow tests. This  is also why it wasn't noticed earlier. => Oo all pre-trained & fine-tuned T5Models still work!

In addition this PR:
- Refactor: clean the code, and remove unnecessarily complicated code
- Remove `n_positions` / `max_position_embeddings` from the config, since T5 is not limited by a  fixed learned position embedding matrix, see: https://github.com/huggingface/transformers/issues/8047

Fixes #8047

Also cc @agemagician for information (I highly doubt though that this will fix the problem we have in your case)

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I have tested both "refactor_t5" and "major_t5_refactor" branches using "T5ForConditionalGeneration".

I didn't re-convert the Tensorflow to Pytorch, since you already told me the conversion process is not the problem.

Doesn't seem it solved our issue yet, but it gives different rubbish output. Maybe, it is a good step into the right direction.

Thanks Patrick.

<|||||>Hi @patrickvonplaten, does this PR effects the checkpoint one gets when calling  ```AutoModel.from_pretrained("t5-3b")```?

 I am investigating why my results with transformers 3.3.1 and T5 changed and encountered this, according to the date seems like it was merged in version 3.5.0. (edit: I see now it was merged in 4.0.0 https://github.com/huggingface/transformers/releases/tag/v4.0.0)

I wonder if this changes the T5 weights/checkpoint I get with my version?