This patch includes two changes to clarify normals in the Python API
First is deprecating `Mesh.calc_normals()`. Since normals are calculated
lazily, this is just misleading because it doesn't actually accomplish
anything.
The second change is making the `MeshVertex.normal` property read-only.
In practice it already was, since the value could be overriten at any
time if code requests updated normals, so this just clarifies that
position and avoids misleading users.
`BKE_mesh_calc_normals` can be removed since it is no longer used.