The `primvars:normals` data is already loaded as custom normals through
the various `bke::mesh_set_custom_normals*` APIs. However, when it came
time to read in the arbitrary primvars we would again read in `normals`
and generate a GN attribute for it. This is unnecessary and wrong as it
can cause issues when re-exporting.
Adds a test that will fail if normals ever slips back in.
Pull Request: https://projects.blender.org/blender/blender/pulls/164130
