remove backend

Author: LiuXiaoxuanPKU

.buildkite/test-pipeline.yaml

@@ -198,8 +198,6 @@ steps:
   - vllm/spec_decode
   - tests/spec_decode
   commands:
-    # See https://github.com/vllm-project/vllm/issues/5152
-    - export VLLM_ATTENTION_BACKEND=XFORMERS
     - pytest -v -s spec_decode/e2e/test_multistep_correctness.py
     - pytest -v -s spec_decode --ignore=spec_decode/e2e/test_multistep_correctness.py