Remove use of VLLM_USE_V1.

[QiliangCui]

Description

Remove VLLM_USE_V1 because it is removed from vllm.main
vllm-project/vllm#28204 removed "VLLM_USE_V1".

FIXES: b/460101498

Tests

Manual launch

# 1 not set VLLM_USE_V1
TPU_BACKEND_TYPE=jax vllm serve Qwen/Qwen2.5-1.5B-Instruct \
 --seed 42 \
 --disable-log-requests \
 --max-num-seqs 512 \
 --max-num-batched-tokens 4096 \
 --tensor-parallel-size 1 \
 --no-enable-prefix-caching \
 --download_dir /mnt/disks/persist \
 --max-model-len 2048

vllm bench serve \
    --backend vllm \
    --model "Qwen/Qwen2.5-1.5B-Instruct" \
    --dataset-name "sonnet" \
    --dataset-path "benchmarks/sonnet_4x.txt" \
    --sonnet-input-len 1200 \
    --sonnet-output-len 1 \
    --num-prompts 1 \
    --request-rate inf \
    --percentile-metrics "ttft,tpot,itl,e2el" \
    --ignore-eos

# 2: still set VLLM_USE_V1=1 making sure it runs as backward compatible. 
VLLM_USE_V1=1 TPU_BACKEND_TYPE=jax vllm serve Qwen/Qwen2.5-1.5B-Instruct \
 --seed 42 \
 --disable-log-requests \
 --max-num-seqs 512 \
 --max-num-batched-tokens 4096 \
 --tensor-parallel-size 1 \
 --no-enable-prefix-caching \
 --download_dir /mnt/disks/persist \
 --max-model-len 2048

vllm bench serve \
    --backend vllm \
    --model "Qwen/Qwen2.5-1.5B-Instruct" \
    --dataset-name "sonnet" \
    --dataset-path "benchmarks/sonnet_4x.txt" \
    --sonnet-input-len 1200 \
    --sonnet-output-len 1 \
    --num-prompts 1 \
    --request-rate inf \
    --percentile-metrics "ttft,tpot,itl,e2el" \
    --ignore-eos

CIT

https://buildkite.com/tpu-commons/tpu-inference-ci/builds/5249

The two broken tests are tracked by other bug as known issue
b/460112990 and b/459771818

[github-actions]

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[wdhongtw]

LGTM. Just a small indent issue and need some noqa comment (for ruff tool) if we want to minimize the line of changes.

[wdhongtw]

LGTM