Remove `alert` when cancelling an upload

I do not see any event that I can override to remove the alert window from popping up.

```javascript
const init = dropzone => {

dropzoneComponent = dropzone;

dropzoneComponent.confirm = function (question, accepted, rejected) {

console.log('...') // never runs

if (window.confirm(question)) {
return accepted();
} else if (rejected != null) {
return rejected();
}
};

return dropzoneComponent;

};
```

No source file or test is named. Start by tracing the upload-cancellation path and how it invokes the confirmation behavior shown in the issue, then determine why the custom confirm function is never reached. Done means cancelling an upload no longer opens the unwanted alert while the intended confirmation behavior remains available.

Written by the indexing model from the issue text.