nixos/modules/config/nsswitch.nix uses `passwdArray` for both `passwd`
and `group`, but when moving this into the sss module in
edddc7c, it didn't get split
appropriately.

nixos/modules/config/nsswitch.nix uses `passwdArray` for both `passwd`
and `group`, but when moving this into the google-oslogin module in
4b71b6f, it didn't get split
appropriately.

nixos/modules/config/nsswitch.nix uses `passwdArray` for both `passwd`
and `group`, but when moving this into the systemd module in
c0995d2, it didn't get split
appropriately.

now that passwdArray and shadowArray aren't used anymore, these can be
folded.

This is now already triggered by the nsswitch module, as we set
system.nssModules.

A disabled nscd breaks nss module loading on NixOS, and systemd without
its nss modules doesn't really work either - instead of silently
disabling its nss modules if nscd is disabled, let the assertion in
nsswitch handle this.

This is all inside a global cfg.enable conditional, so we don't need to
check here again.

Show the config option triggering the assertion, so people don't
necessary lookup the nixpkgs source code.

nixos/nsswitch cleanup nss modules