core: Do not check child freezability when thawing slice
    
    We want thawing operations to still succeed even in the presence of an
    unfreezable unit type (e.g. mount) appearing under a slice after the
    slice was frozen. The appearance of such units should never cause the
    slice thawing operation to fail to prevent potential future repeats of
    https://github.com/systemd/systemd/issues/25356.
    
    (cherry picked from commit b458659a1619337fa83353b5f41bae35d7ad4c69)