[BUG] <title> 去掉默认生成的 async

### Is there an existing issue for this?

- [x] I have searched the existing issues

### Current Behavior

当对接 alova 的时候，因为 alova instance 返回的不是 promise，导致类型提示报错，实际上请求函数的返回值应该由自定义的 request 控制，没必要强制生成 async 函数

### Expected Behavior

_No response_

### Steps To Reproduce

1. openapi-ts-request

### Environment

```markdown
- OS:
```

### Additional context

_No response_