TaskTree: Rename TreeStorage<> -> Storage<>

This addresses the 20th point in the master task below.

Task-number: QTCREATORBUG-28741
Change-Id: I696beda87430fbe637abba8054012fb77516e220
Reviewed-by: hjk <hjk@qt.io>

src/plugins/ios/iosdeploystep.cpp

@@ -107,7 +107,7 @@ public:
 private:
     void cleanup();
 
-    Tasking::GroupItem runRecipe() final;
+    GroupItem runRecipe() final;
 
     void updateDisplayNames();