Remove irrelevant comment in _update_causal_mask

[fxmarty]

Forgot to push a commit to #32227, my bad!

Feel free to merge this one if approved!

[HuggingFaceDocBuilderDev]

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[amyeroberts]

Thanks for handling!

As this changes the if condition, could you run the slow tests for all the model affected?

You'll need to:

• add the run-slow label to the PR
• push a commit with [run_slow] list of model name (space separated list of the model names)