Optimizes ByT5 tokenizer
Author: Narsil
Pull request: yes

# What does this PR do?

- Removes unused logic (actual special tokens are handled by super class

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Fixes #12884


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Models:

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Comments:
There's also a way to speed it up even for 100+ special tokens (by using a single cut pass instead of 100 with byt5) but as mentionned in the issue it's more involved and side effects harder to apprehend.

With this updated version, I am now getting issues with encoding characters which require multiple bytes, e.g. "€" gets tokenized as [8367], where it should be [229, 133, 175]. 

cc @Narsil - I had a similar problem as @gggg8000 previously. Are you sure the optimized ByT5 tokenizer correctly takes single characters that are made of multiple unicode bytes into account?

Oups, I imagined those were covered in tests so I didn't: 

The fix is here: https://github.com/huggingface/transformers/pull/13447