transformer: `declare class` incorrectly preserved as runtime class

Encountered this when bundling Vue via Rolldown.

In
```ts
declare const ReactiveMarkerSymbol: unique symbol

export declare class ReactiveMarker {
  private [ReactiveMarkerSymbol]?: void
}
```

Out
```js
class ReactiveMarker {
  [ReactiveMarkerSymbol];
}
```

Expected
```js
// should be removed entirely
```