Remove Phi 4 Flash configuration workaround

[hmellor]

Now that https://huggingface.co/microsoft/Phi-4-mini-flash-reasoning/discussions/6 and https://huggingface.co/microsoft/Phi-4-mini-flash-reasoning/discussions/7 have been merged, we can remove the workaround.

(The upstream code is currently broken and will be fixed imminently by https://huggingface.co/microsoft/Phi-4-mini-flash-reasoning/discussions/8)

[github-actions]

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[gemini-code-assist]

Code Review

This pull request removes a workaround for the Phi-4-Flash model configuration. The change is predicated on the upstream model configuration on Hugging Face being fixed, as mentioned in the pull request description. This removal simplifies the codebase by eliminating a temporary patch. The change appears correct, and I have no issues to report.