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.
Merge pull request #187 from andrewkaufman/deleteNormalsOnSubdiv

Deleting normals on subdiv meshes converted from Houdini.