systemd: drop reference to deleted `systemd-timesyncd` nixos test

Without the change the eval fails as:

    $ nix build --no-link -f. defaultPkgConfigPackages.libsystemd.tests
    error:
       … while evaluating the attribute 'systemd-timesyncd'

       … in the condition of the assert statement
         at pkgs/os-specific/linux/systemd/default.nix:1023:11:
         1022|           name: value:
         1023|           assert lib.assertMsg (lib.isDerivation value) "${name} is not a derivation";
             |           ^
         1024|           value

       … in the left operand of the OR (||) operator
         at /home/slyfox/dev/git/nixpkgs-master/lib/asserts.nix:50:31:
           49|   # TODO(Profpatsch): add tests that check stderr
           50|   assertMsg = pred: msg: pred || throw msg;
             |                               ^
           51|

       error: attribute 'systemd-timesyncd' missing
       at pkgs/os-specific/linux/systemd/default.nix:1010:13:
         1009|             systemd-sysusers-password-option-override-ordering
         1010|             systemd-timesyncd
             |             ^
         1011|             systemd-timesyncd-nscd-dnssec