nixosTests.installer: Don't wait for the nixos-manual service

The nixos-manual service was removed in #83199 but we missed the check
in this test which prevents the "tested" job from succeeding [0][1].

[0]: https://hydra.nixos.org/build/115472896
[1]: https://hydra.nixos.org/build/115472896/nixlog/94/tail