Remove useless boundary name attributes

src/mesh/MeshEdgeBoundary.cpp

src/mesh/MeshEdgeBoundary.hpp

src/mesh/MeshFaceBoundary.cpp

src/mesh/MeshFaceBoundary.hpp

src/mesh/MeshFlatNodeBoundary.cpp

src/mesh/MeshLineNodeBoundary.cpp

src/mesh/MeshNodeBoundary.cpp

src/mesh/MeshNodeBoundary.hpp

tests/test_MeshFlatEdgeBoundary.cpp

tests/test_MeshFlatFaceBoundary.cpp

tests/test_MeshFlatNodeBoundary.cpp

tests/test_MeshLineFaceBoundary.cpp

tests/test_MeshLineNodeBoundary.cpp

tests/test_MeshNodeBoundary.cpp