[[V0 deprecation]]Remove VLLM_USE_V1 env

[wangxiyuan]

Purpose

This is the last PR to remove VLLM_USE_V1 env totally. Currently, there is no place to use this env and is safe to remote it.

Test Plan

Test Result

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[chatgpt-codex-connector]

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[gemini-code-assist]

Code Review

This pull request completes the removal of the deprecated VLLM_USE_V1 environment variable as part of the V0 deprecation. The changes are straightforward, correctly removing the variable's definition, its usage in tests, and its collection for usage statistics. The modifications are consistent and self-contained. I have reviewed the changes and found no issues of high or critical severity.

[DarkLight1337]

Thanks

[mergify]

Documentation preview: https://vllm--28204.org.readthedocs.build/en/28204/

[wangxiyuan]

@DarkLight1337 For example part, I left mlpspeculator and qwen2.5-omni there instead of removing them, since that I notice there are some PRs to support them in V1 already. These examples can be useful then.

[wangxiyuan]

@DarkLight1337 CI failure looks doesn't relate to this change.

[DarkLight1337]

Retrying flaky test

[mgoin]

It seems we should delete this example rather than allowing it to continue on, if V0 is required cc @ywang96

[DarkLight1337]

See #28204 (comment)

[mgoin]

Ditto, we should delete the example if it doesn't work on V1

[wangxiyuan]

@mgoin I left them there since I notice there are some PRs to support it for V1. So maybe it can be reused later.
mlp_speculator: #21276
qwen2.5-omni: #26334