[Refactor] Fix AttentionMaskBuilder singleton and remove redundant pcp_prefill_mask (#4870)

## What this PR does / why we need it?

This PR fixes the `AttentionMaskBuilder` singleton initialization issue
introduced in PR #4779 and removes the unused `pcp_prefill_mask` field.

### Background

After PR #4779 made `AttentionMaskBuilder` a singleton with `@singleton`
decorator, the class constructor now requires a `device` parameter.
However, two initialization sites were still using the old parameterless
constructor, causing failures.

### Changes

1. **Fix singleton initialization**
- Fixed `AttentionMaskBuilder()` → `AttentionMaskBuilder(self.device)`
in `AscendMLAMetadataBuilder.__init__()`
- Fixed `AttentionMaskBuilder()` → `AttentionMaskBuilder(self.device)`
in `AscendAttentionMetadataBuilder.__init__()`

2. **Remove unused field**
- Removed `pcp_prefill_mask` field from
`AscendPrefillContextParallelMetadata` (never used in codebase)
   - Updated related test assertions

### Related

- Issue #5463
- PR #4779 (Unify all mask generation methods)
- PR #5389 (Make AttentionMaskBuilder singleton)

## Does this PR introduce _any_ user-facing change?

No. This is an internal refactoring.

## How was this patch tested?

- ✅ Local testing: No linter errors
- ✅ Unit tests for attention modules verified
- ⏳ CI pipeline

Signed-off-by: lico67373 <918688502@qq.com>
Co-authored-by: weijinqian0 <1184188277@qq.com>

tests/ut/attention/test_mla_cp.py

@@ -1004,8 +1004,6 @@ class TestAscendMLAImpl(TestBase):
                     [chunk_seqlens, chunk_seqlens], dtype=torch.int32)
                 attn_metadata.prefill.pcp_metadata.head_attn_nomask_seqlens = kv_with_q_head_nomask_seqlens
                 attn_metadata.prefill.pcp_metadata.tail_attn_nomask_seqlens = kv_with_q_tail_nomask_seqlens
-                attn_metadata.prefill.pcp_metadata.pcp_prefill_mask = torch.triu(
-                    torch.ones(10, 10, dtype=torch.float16), 1)
 
                 output = self.impl._forward_prefill(q_nope, q_pe, k_nope, k_pe,
                                                     value, kv_c_and_k_pe_cache,