Remove unused unmountIDFromEnvironment (#7259)

I got rid of the need for this a few months ago.
(cherry picked from commit 30aa84181d)

src/renderers/dom/shared/ReactComponentBrowserEnvironment.js

@@ -27,16 +27,6 @@ var ReactComponentBrowserEnvironment = {
   replaceNodeWithMarkup:
     DOMChildrenOperations.dangerouslyReplaceNodeWithMarkup,
 
-  /**
-   * If a particular environment requires that some resources be cleaned up,
-   * specify this in the injected Mixin. In the DOM, we would likely want to
-   * purge any cached node ID lookups.
-   *
-   * @private
-   */
-  unmountIDFromEnvironment: function(rootNodeID) {
-  },
-
 };
 
 module.exports = ReactComponentBrowserEnvironment;

src/renderers/dom/shared/ReactDOMComponent.js

@@ -23,8 +23,6 @@ var EventConstants = require('EventConstants');
 var EventPluginHub = require('EventPluginHub');
 var EventPluginRegistry = require('EventPluginRegistry');
 var ReactBrowserEventEmitter = require('ReactBrowserEventEmitter');
-var ReactComponentBrowserEnvironment =
-  require('ReactComponentBrowserEnvironment');
 var ReactDOMButton = require('ReactDOMButton');
 var ReactDOMComponentFlags = require('ReactDOMComponentFlags');
 var ReactDOMComponentTree = require('ReactDOMComponentTree');
@@ -1201,7 +1199,6 @@ ReactDOMComponent.Mixin = {
     this.unmountChildren(safely);
     ReactDOMComponentTree.uncacheNode(this);
     EventPluginHub.deleteAllListeners(this);
-    ReactComponentBrowserEnvironment.unmountIDFromEnvironment(this._rootNodeID);
     this._rootNodeID = null;
     this._domID = null;
     this._wrapperState = null;

src/renderers/native/ReactNativeComponentEnvironment.js

@@ -20,15 +20,6 @@ var ReactNativeComponentEnvironment = {
 
   replaceNodeWithMarkup: ReactNativeDOMIDOperations.dangerouslyReplaceNodeWithMarkupByID,
 
-  /**
-   * Nothing to do for UIKit bridge.
-   *
-   * @private
-   */
-  unmountIDFromEnvironment: function(/*rootNodeID*/) {
-
-  },
-
   /**
    * @param {DOMElement} Element to clear.
    */

src/renderers/shared/stack/reconciler/ReactComponentEnvironment.js

@@ -17,13 +17,6 @@ var injected = false;
 
 var ReactComponentEnvironment = {
 
-  /**
-   * Optionally injectable environment dependent cleanup hook. (server vs.
-   * browser etc). Example: A browser system caches DOM nodes based on component
-   * ID and must remove that cache entry when this instance is unmounted.
-   */
-  unmountIDFromEnvironment: null,
-
   /**
    * Optionally injectable hook for swapping out mount images in the middle of
    * the tree.
@@ -42,8 +35,6 @@ var ReactComponentEnvironment = {
         !injected,
         'ReactCompositeComponent: injectEnvironment() can only be called once.'
       );
-      ReactComponentEnvironment.unmountIDFromEnvironment =
-        environment.unmountIDFromEnvironment;
       ReactComponentEnvironment.replaceNodeWithMarkup =
         environment.replaceNodeWithMarkup;
       ReactComponentEnvironment.processChildrenUpdates =