Remove unused removeRootView() method from FabricUIManager

Summary:
FabricUIManager.removeRootView() isn't currently used, removing it from the UIManager interface.

It looks like this is called from JS in paper renderers, but not Fabric, so we should be good to delete it.

Reviewed By: shergin, mdvacca

Differential Revision: D16275118

fbshipit-source-id: b8f3ae1dc7574ce17d8cc9e7fee72ef5dcc9b323

ReactAndroid/src/main/java/com/facebook/react/uimanager/UIManagerModule.java

@@ -421,6 +421,7 @@ public class UIManagerModule extends ReactContextBaseJavaModule
     return tag;
   }
 
+  /** Unregisters a new root view. */
   @ReactMethod
   public void removeRootView(int rootViewTag) {
     mUIImplementation.removeRootView(rootViewTag);