[core] Remove the rootRef properties as unneeded

[oliviertassinari]

I have been digging in the history of those properties. I believe we have been answering the problem with How can I access the DOM element? FAQ question. We do no longer need them. Worse the property coverage of our component was weak. We need a systematic solution to the problem.

Breaking change

-import ReactDOM from 'react-dom';

<IconButton
- rootRef={node => {
-   this.button = ReactDOM.findDOMNode(node);
+ buttonRef={node => {
+   this.button = node;
  }}
>