Duplicate code to compute the normals for TopoDS_Face

There are duplicate code to compute the normals for the TopoDS_Face:
1. void StdPrs_ToolShadedShape::Normal (const TopoDS_Face& theFace,
                                     Poly_Connect& thePolyConnect,
                                     TColgp_Array1OfDir& theNormals)

2. In StlTransfer.cxx, also have the function to compute normals for surface:
   static void Normal(const TopoDS_Face& aFace,
           Poly_Connect& pc,
           TColgp_Array1OfDir& Nor)
   Though the normals are calcuted for the TopoDS_Face, but when output them to STL file, do not use these normals. The normals in STL file are normal of the triangle, not the vertex. So the function Normal() in StlTransfer.cxx can be removed.