vLLM integration page links to a GitHub repo supporting Haystack 1.x only and should be removed

https://github.com/deepset-ai/haystack-integrations/blob/main/integrations/vllm.md links to https://github.com/LLukas22/vLLM-haystack-adapter which only supports Haystack 1.x. We can actually remove the repo because only vllm and haystack need to be installed but no separate package is needed for the two packages to work together.