Remove Qwen Omni workaround that's no longer necessary

[hmellor]

This PR removes a workaround for Qwen Omni models which is no longer necessary on newer Transformers versions.

There have been several Transformers releases since huggingface/transformers#37517 (the condition for removing this workaround) was merged and vLLM is currently on the latest version of Transformers.

[github-actions]

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[gemini-code-assist]

Code Review

This pull request removes a workaround for Qwen Omni models, which is no longer necessary with recent versions of the transformers library. The change is a simple deletion of the special-casing logic, which improves code clarity and maintainability. The change is correct and beneficial, and I did not find any issues of high or critical severity.