Editing an imported module causes duplicate display

[mbostock]

#1416 introduced a bug where old displays are not cleaned up when imported modules change. To reproduce, create a JavaScript fenced code block like so:

```js echo
import {test} from "./test.js";

display(test);
```

Then create the test.js like so:

export const test = 2;

Edit test.js and you will see the bug: