### What this PR does / why we need it?
Remove unused vanilla attn code.
### Does this PR introduce _any_ user-facing change?
NA
- vLLM version: v0.12.0
- vLLM main:
ad32e3e19c
Signed-off-by: zzzzwwjj <1183291235@qq.com>
Co-authored-by: wangxiyuan <wangxiyuan1007@gmail.com>