[Cleanup] Remove unused attn_metadata parameter from Proposer classes (#4862)

The `attn_metadata` is not used by any draft proposer, so we can remove
it.


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

---------

Signed-off-by: Jade Zheng <zheng.shoujian@outlook.com>

vllm_ascend/spec_decode/ngram_proposer.py

@@ -38,7 +38,6 @@ class NgramProposer(VllmNgramProposer, Proposer):
                            positions=None,
                            num_scheduled_tokens=None,
                            hidden_states=None,
-                           attn_metadata=None,
                            aux_hidden_states=None) -> list[list[int]]:
         valid_ngram_requests = []
         for i, sampled_ids in enumerate(valid_sampled_token_ids):