Remove V0 attention backends (#25351)

Signed-off-by: Woosuk Kwon <woosuk.kwon@berkeley.edu>

tests/models/test_initialization.py

@@ -78,9 +78,8 @@ def can_initialize(model_arch: str, monkeypatch: pytest.MonkeyPatch,
             return
 
         if model_arch in ("Phi4FlashForCausalLM", "MotifForCausalLM"):
-            # Phi4FlashForCausalLM and MotifForCausalLM
-            # only supports DIFFERENTIAL_FLASH_ATTN backend
-            m.setenv("VLLM_ATTENTION_BACKEND", "DIFFERENTIAL_FLASH_ATTN")
+            pytest.skip(
+                "Differential Flash Attention backend has been removed.")
         if model_arch == "GptOssForCausalLM":
             # FIXME: A hack to bypass FA3 assertion because our CI's L4 GPU
             # has cc==8.9 which hasn't supported FA3 yet. Remove this hack when