[V0 Deprecation] Remove VLLM_USE_V1 from tests (#26341)

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

tests/distributed/test_pipeline_parallel.py

@@ -307,7 +307,6 @@ def _compare_tp(
     if distributed_backend == "ray":
         # For V1, test Ray Compiled Graph for all the tests
         pp_env = {
-            "VLLM_USE_V1": "1",
             "VLLM_USE_RAY_COMPILED_DAG": "1",
             "VLLM_USE_RAY_SPMD_WORKER": "1",
             "VLLM_USE_RAY_COMPILED_DAG_NCCL_CHANNEL": "1",
@@ -316,15 +315,11 @@ def _compare_tp(
         # terminate because of a Ray Compiled Graph issue.
         common_args.append("--disable-frontend-multiprocessing")
     elif distributed_backend == "mp":
-        pp_env = {
-            "VLLM_USE_V1": "1",
-        }
+        pp_env = None
     else:
         pp_env = None
 
-    tp_env = {
-        "VLLM_USE_V1": "1",
-    }
+    tp_env = None
 
     pp_args = [
         *common_args,