[Misc] Remove redundant attention var constants (#29650)

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

tests/models/quantization/test_fp8.py

@@ -11,7 +11,6 @@ import pytest
 from tests.quantization.utils import is_quant_method_supported
 from vllm.attention.utils.fa_utils import flash_attn_supports_fp8
 from vllm.platforms import current_platform
-from vllm.utils import STR_BACKEND_ENV_VAR
 from ..utils import check_logprobs_close
 
 
@@ -76,7 +75,7 @@ def test_models(
 
     with monkeypatch.context() as m:
         m.setenv("TOKENIZERS_PARALLELISM", "true")
-        m.setenv(STR_BACKEND_ENV_VAR, backend)
+        m.setenv("VLLM_ATTENTION_BACKEND", backend)
 
         MAX_MODEL_LEN = 1024
         NUM_LOG_PROBS = 8