[BugFix][Spec Decode] Remove obsolete sequence length update (#15105)

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

This PR removes the unused `_ascend_update_seq_lens` method and its
invocation in `_run_model` within `AscendAutoRegressiveSpeculator`.

The method runs after the draft model execution has completed, and the
generated `seq_len_list` is not consumed by the subsequent speculative
decoding flow. Sequence lengths needed by draft attention are already
prepared through the existing attention metadata update path, so this
additional update is unnecessary.

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

No.

### How was this patch tested?

- `python -m py_compile
vllm_ascend/worker/v2/spec_decode/autoregressive/speculator.py`
- `git diff --check`

Signed-off-by: jiajinzhu2 <jiajinzhu@huawei.com>