Fix vLLM v1 compatibility

[Atry]

Don't delete model_executor if it does not exist, e.g. when VLLM_USE_V1 is set.

See https://docs.vllm.ai/en/stable/serving/env_vars.html and https://github.com/vllm-project/vllm/blob/main/vllm/v1/engine/llm_engine.py

[codspeed-hq]

CodSpeed Performance Report

Merging #1121 will not alter performance

_{Comparing Atry:patch-1 (9df6612) with main (1b6c101)}

Summary

✅ 1 untouched benchmarks

[plaguss]

Thanks for the fix!