Fix ref behavior for remounting

I'm thinking that setting up `this.refs` in `mountComponent` is better than `construct`. Followed the same pattern as `ReactComponent.Mixin` and nulling out
the value in `construct` and setting it to its initial value in `mountComponent`.

src/core/ReactCompositeComponent.js

@@ -978,7 +978,6 @@ var ReactCompositeComponentMixin = {
     this._renderedComponent = null;
 
     ReactComponent.Mixin.unmountComponent.call(this);
-    ReactOwner.Mixin.unmountComponent.call(this);
 
     // Some existing components rely on this.props even after they've been
     // destroyed (in event handlers).

src/core/ReactOwner.js

@@ -150,10 +150,6 @@ var ReactOwner = {
      */
     detachRef: function(ref) {
       delete this.refs[ref];
-    },
-
-    unmountComponent: function() {
-      this.refs = null;
     }
 
   }