nixos-generate-config: Don't suggest setting a uid

This hasn't been needed for a long time, even when `mutableUsers =
false`. Setting a uid manually is potentially risky since it could
collide with non-declarative user accounts. (We do check for
collisions between declarative accounts.)

nixos/modules/installer/tools/nixos-generate-config.pl

@@ -641,7 +641,6 @@ $bootLoaderConfig
   # Define a user account. Don't forget to set a password with ‘passwd’.
   # users.users.jane = {
   #   isNormalUser = true;
-  #   uid = 1000;
   #   extraGroups = [ "wheel" ]; # Enable ‘sudo’ for the user.
   # };