[refactor] Remove unnecessary attributes from set_ascend_forward_context (#5204)

### What this PR does / why we need it?
Remove unnecessary attributes from set_ascend_forward_context
1.prefetch_stream
2.weight_prefetch_method
### Does this PR introduce _any_ user-facing change?
no
### How was this patch tested?

- vLLM version: v0.12.0
- vLLM main:
ad32e3e19c

Signed-off-by: Wang Kunpeng <1289706727@qq.com>

tests/ut/quantization/test_w8a8.py

@@ -63,21 +63,18 @@ class TestAscendW8A8LinearMethod(TestBase):
         self.assertEqual(params['weight_scale'].shape, (10, 1))
         self.assertEqual(params['weight_offset'].shape, (10, 1))
 
-    @patch("vllm_ascend.quantization.w8a8.get_forward_context")
+    @patch("vllm_ascend.quantization.w8a8.get_weight_prefetch_method")
     @patch("torch.ops.vllm.quantize")
     @patch("torch_npu.npu_quant_matmul")
     def test_apply_with_x_not_int8(self, mock_npu_quant_matmul, mock_quantize,
-                                   mock_get_forward_context):
+                                   mock_get_weight_prefetch_method):
         layer = MagicMock()
         layer.aclnn_input_scale = 0.1
         layer.aclnn_input_offset = 0.2
         layer.weight = torch.randn(128, 256)
         layer.deq_scale = 0.3
 
-        mock_forward_context = MagicMock()
-        mock_get_forward_context.return_value = mock_forward_context
-        mock_weight_prefetch_method = MagicMock()
-        mock_forward_context.weight_prefetch_method = mock_weight_prefetch_method
+        mock_get_weight_prefetch_method.return_value = MagicMock()
 
         x = torch.randn(32, 128)
         bias = torch.randn(256)