[bugfix][ACLGraph][MTP] deletes cudagraph_batch_sizes in MtpProposer (#5183)

### What this PR does / why we need it?
This PR deletes `cudagraph_batch_sizes` in `MtpProposer` and reuses the
one in `NPUModelRunner`.

During our deployment of DeepSeek-V3.2 with MTP across machines 2P2D and
conducting AISBench stress testing, an error occurred (see below). After
investigation, we found that
`compilation_config.cudagraph_capture_sizes` is modified by
`adjust_cudagraph_sizes_for_spec_decode` in `NPUModelRunner`. This
modification only updates `cudagraph_batch_sizes` in `NPUModelRunner`
but is not synchronized to `MtpProposer`. After discussion (CC @yiz-liu)
, we believe it is unnecessary to maintain `cudagraph_batch_sizes` in
`MtpProposer`; it should directly use the variable from
`NPUModelRunner`.

- vLLM version: v0.12.0
- vLLM main:
ad32e3e19c

---------

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

tests/ut/spec_decode/test_mtp_proposer.py

@@ -97,7 +97,6 @@ class TestMtpProposer:
         proposer = MtpProposer(vllm_config, torch.device("cpu"), runner)
 
         assert proposer.use_aclgraph is True
-        assert proposer.cudagraph_batch_sizes == [1, 2, 4, 8]
 
     @patch("vllm.config.get_layers_from_vllm_config")
     @patch("vllm_ascend.spec_decode.mtp_proposer.get_model_loader")