[NUI] remove unnecessary code to assign children layout (#1554)

[NUI] remove unnecessary code to assign children layout (#1554)

layout is set by `LayoutGroup.AddChildToLayoutGroup()`

1.When layout is set before child added
```
Tizen.NUI.LayoutGroup.AddChildToLayoutGroup() Line 164
Tizen.NUI.LayoutGroup.OnChildAddedToOnwer() Line 265
Tizen.NUI.BaseComponent.View.Add() Line 152
```

2.When layout is set after child added
```
Tizen.NUI.LayoutGroup.AddChildToLayoutGroup() Line 164
Tizen.NUI.LayoutGroup.OnAttachedToOwner() Line 466
Tizen.NUI.LayoutItem.AttachToOnwner() Line 168
Tizen.NUI.BaseComponentsView.SetLayout() Line 110
Tizen.NUI.BaseComponentsView.set_Layout() Line 2214

```

This means children's Layout is alreay assinged on `LayoutController.Process()`.
So, we don't need to assign layout again.