pkgset: Fix running without any koji tags
    
    All places in the code assume the option to not be required except for
    this one line.
    
    An obsolete comment is removed as well.
    
    Relates: https://pagure.io/releng/failed-composes/issue/477
    Signed-off-by: Lubomír Sedlář <lsedlar@redhat.com>