[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>

tests/ut/attention/test_mla_v1.py

@@ -137,7 +137,6 @@ class TestAscendMLADecodeMetadata(TestBase):
         seq_lens_list = [2, 3]
         attn_mask = None
         cp_seq_len = torch.tensor([2, 3])
-        batch_seq_mask = torch.tensor([[1, 1, 0, 0], [1, 1, 1, 0]])
 
         metadata = AscendMLADecodeMetadata(input_positions=input_positions,
                                            block_table=block_table,
@@ -145,8 +144,7 @@ class TestAscendMLADecodeMetadata(TestBase):
                                            max_seq_lens=max_seq_lens,
                                            seq_lens_list=seq_lens_list,
                                            attn_mask=attn_mask,
-                                           cp_seq_len=cp_seq_len,
-                                           batch_seq_mask=batch_seq_mask)
+                                           cp_seq_len=cp_seq_len)
 
         self.assertIs(metadata.input_positions, input_positions)
         self.assertIs(metadata.block_table, block_table)
@@ -155,7 +153,6 @@ class TestAscendMLADecodeMetadata(TestBase):
         self.assertEqual(metadata.seq_lens_list, seq_lens_list)
         self.assertIsNone(attn_mask)
         self.assertIs(metadata.cp_seq_len, cp_seq_len)
-        self.assertIs(metadata.batch_seq_mask, batch_seq_mask)
 
 
 class TestAscendMLAMetadata(TestBase):