Support control array portals in ArrayHandleCompositeVector.

Previously, the ArrayHandleCompositeVector had a separate implementation of ArrayPortal for the control and execution environments. Because I was lazy when I implemented it, the control version did not support Get.

Since originally implementing this class, VTK-m now allows defining methods that are declared as working in both control and execution environments (VTKM_EXEC_CONT_EXPORT) but only work in one or the other depending on methods of templated subclasses they call. Thus, solve this problem by simply removing the control version of the portal and use the same portal for both.