rm vanilla attn (#4558)

### 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:
vllm-project/vllm@ad32e3e

Signed-off-by: zzzzwwjj <[email protected]>
Co-authored-by: wangxiyuan <[email protected]>

Author: zzzzwwjj