[V1] Remove additional_config check

[wangxiyuan]

additional_config is contained in vllm_config and can be used by custom platform. It doesn't affect any V1 feature or function and can be used by V1 correctly. This PR drop the related check for V1 engine.

[github-actions]

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[russellb]

Hey @robertgshaw2-redhat, I assume this check was intentional. Can you take a look?

[wangxiyuan]

@robertgshaw2-redhat hi, may I ask for your review? we test V1 engine with additional_config. It works well. Anything I missed?

[robertgshaw2-redhat]

Good by me. I was not sure what this was for, so I disabled out of an abundance of caution.

[robertgshaw2-redhat]

Thanks for the PR!

[wangxiyuan]

Test failure with lora error. Looks doesn't relate to this PR