pkgset: Remove reuse file when packages are not signed
    
    In such case we never want to reuse the pkgset, as it risks leaking
    unsigned packages. Safest option is to remove the file completely.
    
    Fixes: https://pagure.io/pungi/issue/1480
    JIRA: RHELCMP-3720
    Signed-off-by: Lubomír Sedlář <lsedlar@redhat.com>