[chore] remove warning about injectCSS.
[chore] remove warning about injectCSS.

`injectCSS` was deprecated since version 0.5.0.
Time to finally remove it.

reactjs/react-modal,springload/react-modal,reactjs/react-modal,bluSCALE4/react-modal,rackt/react-modal,springload/react-modal,bluSCALE4/react-modal,springload/react-modal,bluSCALE4/react-modal,diasbruno/react-modal,rackt/react-modal,diasbruno/react-modal,reactjs/react-modal,diasbruno/react-modal