Remove misleading comment about RootTag type not being opaque

Summary: Changelog: [Internal]

Reviewed By: lunaleaps

Differential Revision: D40902247

fbshipit-source-id: 5ed8f69bb46e35d5250bd04898765ead500d7db6

Author: rshest

Libraries/ReactNative/RootTag.js

@@ -10,7 +10,6 @@
 
 import * as React from 'react';
 
-// TODO: Make this into an opaque type.
 export opaque type RootTag = number;
 
 export const RootTagContext: React$Context<RootTag> =