Fix warning on FileWidget (rjsf-team#842)

Author: lauriii

src/components/widgets/FileWidget.js

@@ -60,10 +60,6 @@ function extractFileInfo(dataURLs) {
 }
 
 class FileWidget extends Component {
- defaultProps = {
- multiple: false,
- };
-
  constructor(props) {
  super(props);
  const { value } = props;