Add remove_nested_key() to __all__ for utils
Add remove_nested_key() to __all__ for utils

I neglected to do this when I added the method in f1fe627. I don't believe I ever use `from .utils import *`, but having all methods listed in `__all__` is important in case anyone ever does--although they shouldn't. Ever.

Actually, I think Sphinx does an `import *` when using `auto module`.

dirn/Simon,dirn/Simon