remove ax argument from ROC constructor

[edublancas]

we found an issue in the docs (#231) that was patched by passing ax to the plot method of ROC (#250).

I just realized the ROC constructor also has an ax argument, which shouldn't be the case and we should remove it since ax should only be in the plot method.

I took a quick glance at the code and it looks like the ax in the constructor isn't doing anything so we can remove it. and I don't think people are using this argument so we can remove it silently (no need to deprecate it)