Deleting normals on subdiv meshes converted from Houdini.

Subdivision meshes should not have normals, but they tend to get added accidentally when dealing with many meshes in a single SOP, as that would extend the normals attribute to all meshes. We force N to be excluded when converting subdiv (catmullClark) meshes directly in the FromHoudiniPolygonsConverter. I considered making an optional parameter to keep normals in this case, but since most uses of the converters take default parameter values, I thought it wasn't necessarily worth while. I can add the option of there are strong opinions otherwise though.