[bugfix] fix the complex and potentially problematic generate_kv_idx. (#5957)

### What this PR does / why we need it?
In long-sequence scenarios, the chunked-prefill component may encounter
dimension misalignment issues, which previously occurred during
precision testing on the code_generate_lite dataset. This PR removes
redundant computations and instead derives the value using existing
results and straightforward calculations.
- vLLM version: v0.13.0
- vLLM main:
2c24bc6996

Signed-off-by: QiuChunshuo <qiuchunshuo@huawei.com>

vllm_ascend/attention/utils.py

@@ -70,8 +70,6 @@ class AscendPrefillContextParallelMetadata:
 
     pcp_allgather_restore_idx: torch.Tensor = None
 
-    cp_kv_recover_idx_for_chunk: torch.Tensor = None
-
     num_actual_tokens_pcp_padded: int = 0
 
     num_computed_tokens_of_pcp_dcp: list[list[list[int]]] | None = None