Fix double-firing submit events (#12877)

We were adding a listener at the root when we weren't meant to. Blames to e96dc140599363029bd05565d58bcd4a432db370. This now alerts once (at FORM) instead of twice (at FORM, #document): ``` var Hello = class extends React.Component { render() { return ( <form onSubmit={(e) => {e.preventDefault(); alert('hi ' + e.nativeEvent.currentTarget.nodeName);}}> <button>hi</button> </form> ); } }; ```