Remove outdated comment

It was fixed in facebook#8451.

Author: gaearon

src/renderers/dom/__tests__/ReactDOMProduction-test.js

@@ -180,9 +180,6 @@ describe('ReactDOMProduction', () => {
     expect(function() {
       class Component extends React.Component {
         render() {
-          // FIXME: undefined is a valid return type in Fiber, at least in the
-          // current implementation. It's treated as empty. I don't know why we
-          // sometimes get false positives.
           return undefined;
         }
       }