[Gemma, Llama] Remove unused attribute

[NielsRogge]

What does this PR do?

This PR removes the spaces_between_special_tokens attribute from GemmaTokenizer and LlamaTokenizer since that attribute doesn't do anything. This is because the _decode method in tokenization_utils.py doesn't make use of this attribute, it only accepts a flag which defaults to True.

Alternatively, we could make sure that that attribute actually does something, by checking in the _decode method whether the tokenizer has it set. Up for discussion

[HuggingFaceDocBuilderDev]

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[ArthurZucker]

We should probably update the _decode function to use it TBH!

[ArthurZucker]

Or always set it to False and just deprecate it. So many people are confused by it

[ArthurZucker]

Both work hand in hand, we set it to True for old models

[github-actions]

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