remove unnecessary definitions of const static variables

MeshLib/Elements/HexRule8.cpp

MeshLib/Elements/LineRule2.cpp

MeshLib/Elements/LineRule3.cpp

MeshLib/Elements/PrismRule6.cpp

MeshLib/Elements/PyramidRule5.cpp

MeshLib/Elements/QuadRule4.cpp

MeshLib/Elements/QuadRule8.cpp

MeshLib/Elements/QuadRule9.cpp

MeshLib/Elements/TetRule4.cpp

MeshLib/Elements/TriRule3.cpp