documentation: Don't set localSystem which doesn't exist

[roberth]

This is safe to remove, because it is not referenced anywhere in
nix-darwin.
It should have been discovered way earlier, but a bug in the
module system has allowed this value to be defined until
NixOS/nixpkgs@fd75dc8

[marsam]

Thanks a lot! I can confirm it fixes the evaluation

[LnL7]

Thanks.

Having these kind of breaking api changes are a bit problematic for projects outside of nixos however. It would be great if this was tied to the release cycle for example.

[roberth]

This only happened with nixpkgs unstable. Users on nixpkgs 20.03 were not affected. Does that count as "tied to the release cycle" or did you have something else in mind?

[LnL7]

Well I would consider unstable packages and unstable apis separate things, as an example we generally don't rename packages on unstable either without adding an alias for compatibility. As it stands there's no way to provide a transition window while still allowing rolling package updates, which is what most people use in practice.