[main] remove dbo code (#3712)

### What this PR does / why we need it?
Remove codes of dbo.
Currently, vLLM has supported dbo with pr:
https://github.com/vllm-project/vllm/pull/23693.

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

### How was this patch tested?

- vLLM version: v0.11.0rc3
- vLLM main:
17c540a993

Signed-off-by: zzzzwwjj <1183291235@qq.com>

vllm_ascend/attention/utils.py

@@ -91,8 +91,6 @@ class AscendCommonAttentionMetadata:
 
     attn_state: Any = None
 
-    enable_dbo_across_dp: bool = False
-
     is_only_prefill: bool = False
 
     graph_pad_size: int = -1