Kill NavigationReducers

Summary:
For navigation actions at high level, reducers from NavigationReducers does not
know anything about the app-specific state thus people won't use these reducers.
Instead, people should build their own reducers.

There are a lot of good libraries available that help people to reducing things if that's
what they really need.

At the low level, for navigation state changes that don't involve app-specific state,
`NavigationStateUtils` should server that kind of need.

`NavigationReducers` serves little benefit cause it does not know the app state, it does
not know how to traverse the navigation states which can be a tree, a list or a map.

That said, we hold no interest in owning in the core navigation library.

Reviewed By: ericvicenti

Differential Revision: D3372910

fbshipit-source-id: 797382b46e7d64b7ad578b51dd37e2b941faa83d

Examples/UIExplorer/NavigationExperimental/NavigationCardStack-example.js

@@ -80,7 +80,7 @@ class YourApplication extends React.Component {
     switch (type) {
       case 'push':
         // push a new route.
-        const route = {key: Date.now()};
+        const route = {key: 'route-' + Date.now()};
         navigationState = NavigationStateUtils.push(navigationState, route);
         break;