nixos/sssd remove redundant condition

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

nixos/modules/services/misc/sssd.nix

@@ -69,7 +69,7 @@ in {
         mode = "0400";
       };
 
-      system.nssModules = optional cfg.enable pkgs.sssd;
+      system.nssModules = pkgs.sssd;
       system.nssDatabases = {
         group = [ "sss" ];
         passwd = [ "sss" ];