Drop the user_notifications configuration key
    
    That key was present only in the tests and not used anywhere else
    in the code.
    The toddler distgit-bugzilla-sync does not warn any user that their
    account are incorrectly set-up. This is achived by the toddler:
    packager-without-bugzilla which runs daily.
    
    Signed-off-by: Pierre-Yves Chibon <pingou@pingoured.fr>