CLN: prune unreachable code
I'm all for simplifying code, but I'm not really seeing how any of the code deleted was "unreachable".  Is there a previous PR or issue that impelled this? > but I'm not really seeing how any of the code deleted was "unreachable"

For private/internal code, we can just look at all the places where it is called and determine that e.g. ShallowMixin._shallow_copy always has `obj` passed. thanks @jbrockmendel 