[NUI] Remove unused conditional expression in LayoutGroup

[NUI] Remove unused conditional expression in LayoutGroup

Since AddChildToLayoutGroup is called only if view's Layout is set, the
conditional expression is meaningless.

Owner.LayoutSet is always true, so this is unnecessary code.

GetType() is always Tizen.NUI.LayoutGroup, so the intended code might be
child.GetType() == typeof(View).

The intended code is incorrect, because any class, derived from View, is
also required to set default Layout if it does not set Layout.

If the class, derived from View, uses its own Layout instead of the
default Layout, then the class already sets its own Layout, so
child.Layout cannot be null.