fix(xpu): remove XPU_CCL_BACKEND

Signed-off-by: dbyoung18 <yang5.yang@intel.com>

Author: dbyoung18

vllm/platforms/__init__.py

@@ -138,30 +138,18 @@ def xpu_platform_plugin() -> Optional[str]:
     logger.debug("Checking if XPU platform is available.")
     try:
         # installed IPEX if the machine has XPUs.
-        # detect dist_backend
-        import os
-
         import intel_extension_for_pytorch  # noqa: F401
         import torch
         if supports_xccl():
-            default_backend = "xccl"
+            dist_backend = "xccl"
         else:
-            logger.debug("xccl is not available.")
-            default_backend = "ccl"
-        detect_backend = os.getenv("XPU_CCL_BACKEND", default_backend)
-
-        if detect_backend not in ["xccl", "ccl"]:
-            raise ValueError(
-                f"Unknown {detect_backend} backend for XPU platform.")
-
-        if detect_backend == "ccl":
-            logger.debug("Checking if ccl is available.")
+            dist_backend = "ccl"
             import oneccl_bindings_for_pytorch  # noqa: F401
 
         if hasattr(torch, 'xpu') and torch.xpu.is_available():
             is_xpu = True
             from vllm.platforms.xpu import XPUPlatform
-            XPUPlatform.dist_backend = detect_backend
+            XPUPlatform.dist_backend = dist_backend
             logger.debug("Confirmed %s backend is available.",
                          XPUPlatform.dist_backend)
             logger.debug("Confirmed XPU platform is available.")