BUG: Removed crashing vtkMRMLSliceNode::SetSliceToRAS method

Using vtkCxxSetObjectMacro on a vtkSmartPointer-based variable always crashes. There was an option to either change SliceToRAS to a simple pointer and still allow users to set the pointer from outside, but object ownership would not have been very clear, several NULL-pointer checks should have been included; and already ->GetSliceToRAS()->DeepCopy(...) us used in Slicer core to modify the variable everywhere in the Slicer core and it is better not to have different mechanisms for the same purpose.

git-svn-id: http://svn.slicer.org/Slicer4/trunk@25199 3bd1e089-480b-0410-8dfb-8563597acbee