remove unused tarballs from sources
    
    The sources file should only contain entries which are used in the
    current spec file.  Remove the unused entries to avoid fedpkg from
    reporting 'Not downloading unused ...' for older releases, e.g.:
    
        $ fedpkg sources
        Not downloading unused logwatch-7.5.4.tar.gz
        Not downloading unused logwatch-7.5.5.tar.gz
        Not downloading unused logwatch-7.5.6.tar.gz
        Not downloading unused logwatch-7.6.tar.gz
        Not downloading unused logwatch-7.7.tar.gz
        Downloading logwatch-7.8.tar.gz