[V0 Deprecation] Remove VLLM_USE_V1 from docs and scripts (#26336)

Signed-off-by: DarkLight1337 <tlleungac@connect.ust.hk>

.buildkite/nightly-benchmarks/scripts/run-performance-benchmarks.sh

@@ -454,11 +454,6 @@ main() {
   fi
   check_hf_token
 
-  # Set to v1 to run v1 benchmark
-  if [[ "${ENGINE_VERSION:-v0}" == "v1" ]]; then
-    export VLLM_USE_V1=1
-  fi
-
   # dependencies
   (which wget && which curl) || (apt-get update && apt-get install -y wget curl)
   (which jq) || (apt-get update && apt-get -y install jq)