[Fix] Remove unnecessary NPU synchronization in MTP proposer (#4325)

### What this PR does / why we need it?
Remove unnecessary NPU synchronization in MTP proposer to improve
performances.

Removing this synchronization point improves pipeline efficiency by
allowing for better overlap between CPU and NPU operations. A more
proper one is already implemented in #4233

### Does this PR introduce _any_ user-facing change?
None.

### How was this patch tested?
None.


- vLLM version: v0.11.0
- vLLM main:
vllm-project/vllm@2918c1b

Signed-off-by: Yizhou Liu <[email protected]>

Author: yiz-liu

vllm_ascend/spec_decode/mtp_proposer.py

@@ -886,7 +886,6 @@ def _propose(
                 attn_metadata_i.decode.max_seq_lens = min(
                     attn_metadata_i.decode.max_seq_lens,
                     self.runner.model_config.max_model_len)
-            torch.npu.synchronize()
 
         # mtp>1: [batch_size, k]
         draft_token_ids = torch.stack(draft_token_ids_list, dim=1)