remove redundant cls alias on Windows

It's already a magic, and the duplicate alias was failing with Invalid alias.

The conditions for the alias and magic differ, which is definitely wrong:

- `sys.platform == 'win32'` for the magic
- `os.name == 'nt'` for the alias

but I have no idea which one is preferable.

We could also invert this patch, so only the alias is defined and not the magic, but obviously we shouldn't have both.