No unused attribute warning for attributes without "bs." prefix

I came across this while cleaning up the `bs.` prefix handling.

```
@bs.string
let x = 42
```

gives

```
[E] Line 1, column 0:
Unused attribute: bs.string
This means such annotation is not annotated properly. 
for example, some annotations is only meaningful in externals 
```

whereas

```
@string
let x = 42
```

gives no error.