[Misc] Remove CP Redundant Variables after FIA operator enables for CANN 8.5 (#6013)

### What this PR does / why we need it?
PCP/DCP splits the kv-cache onto different cards. After introducing the
parameter cp-kv-cache-interleave-size, the first size tokens will be
cached at Card 0, and so on.
However, if there are too few tokens, some cards will not store the
key-value pairs, resulting in values ​​of 0, corrupted values, and
precision issues. Currently, additional operations are introduced to
avoid this precision problem.

After we integrate FIA operator in mla_cp._forward_decode and CANN
updates to 8.5.0, we now can remove these additional operations.
### Does this PR introduce _any_ user-facing change?

### How was this patch tested?
passed all CI by CANN 8.5.0
- vLLM version: v0.13.0
- vLLM main:
2c24bc6996

Signed-off-by: dsxsteven <dsxsteven@sina.com>
Signed-off-by: dsxsteven <36877507+dsxsteven@users.noreply.github.com>

vllm_ascend/attention/mla_v1.py

@@ -134,7 +134,6 @@ class AscendMLADecodeMetadata:
     sin: torch.Tensor = None
     cos: torch.Tensor = None
     cp_seq_len: torch.Tensor = None
-    batch_seq_mask: torch.Tensor = None
 
 
 @dataclass
@@ -577,7 +576,7 @@ class AscendMLAMetadataBuilder(MLACommonMetadataBuilder[AscendMLAMetadata]):
             self.block_table = self.block_table[:self.graph_pad_size, ...]
         seq_lens_list = self.seq_lens.tolist()
 
-        cp_seq_len, batch_seq_mask = None, None
+        cp_seq_len = None
 
         if self.graph_pad_size > num_reqs:
             if self.speculative_config.disable_padded_drafter_batch:
@@ -638,8 +637,7 @@ class AscendMLAMetadataBuilder(MLACommonMetadataBuilder[AscendMLAMetadata]):
             actual_seq_lengths_q=actual_seq_lengths_q,
             sin=sin[:self.num_decode_tokens, ...],
             cos=cos[:self.num_decode_tokens, ...],
-            cp_seq_len=cp_seq_len,
-            batch_seq_mask=batch_seq_mask)
+            cp_seq_len=cp_seq_len)
         return decode_metadata
 
     def build_for_graph_capture(