rm vanilla attn

[zzzzwwjj]

What this PR does / why we need it?

Does this PR introduce any user-facing change?

How was this patch tested?

• vLLM version: v0.11.2
• vLLM main: https://github.com/vllm-project/vllm/commit/v0.11.2

[github-actions]

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[gemini-code-assist]

Code Review

This pull request removes the fallback 'vanilla' attention implementation (vanilla_chunked_prefill), which was a workaround for cases with head_size=192. This is a positive cleanup that improves maintainability. However, the test that covered this specific head size has also been removed, which introduces a gap in test coverage. I have added a comment recommending the addition of a new test to verify that the main attention path now correctly handles this edge case, which is crucial to prevent potential regressions.