nixos/nscd: don't need to specify username

Thanks to @arianvp for pointing out that when DynamicUser is true,
systemd defaults the value of User to be the name of the unit, which in
this case is already "nscd".

nixos/modules/services/system/nscd.nix

@@ -55,7 +55,6 @@ in
         serviceConfig =
           { ExecStart = "@${pkgs.glibc.bin}/sbin/nscd nscd";
             Type = "forking";
-            User = "nscd";
             DynamicUser = true;
             RuntimeDirectory = "nscd";
             PIDFile = "/run/nscd/nscd.pid";