Revert "nixos: rename system.{stateVersion,defaultChannel} -> system.nixos.\1"

This reverts commit 095fe5b4.

Pointless renames considered harmful. All they do is force people to
spend extra work updating their configs for no benefit, and hindering
the ability to switch between unstable and stable versions of NixOS.

Like, what was the value of having the "nixos." there? I mean, by
definition anything in a NixOS module has something to do with NixOS...