clean 0.15.0 support (#6852)

Clean up vllm 0.15.0 related code

- vLLM version: v0.16.0
- vLLM main:
15d76f74e2

Signed-off-by: wangxiyuan <wangxiyuan1007@gmail.com>

vllm_ascend/sample/rejection_sampler.py

@@ -19,7 +19,6 @@ from vllm_ascend.ops.triton.reject_sample import (
     sample_recovered_tokens_kernel,
 )
 from vllm_ascend.sample.sampler import apply_top_k_top_p
-from vllm_ascend.utils import vllm_version_is
 
 
 def apply_sampling_constraints(
@@ -167,10 +166,7 @@ def rejection_sample(
             return output_token_ids
 
     # Compute probability distribution from target logits.
-    if vllm_version_is("0.15.0"):
-        target_probs = target_logits
-    else:
-        target_probs = target_logits.softmax(dim=-1, dtype=torch.float32)
+    target_probs = target_logits.softmax(dim=-1, dtype=torch.float32)
     assert target_probs.is_contiguous()
 
     # Generate uniform probabilities for rejection sampling.