krb5: refactor removal of krb5info files
    
    Currently a persistent offline callback removes the krb5info files for
    the configured main domain and those files were removed by a SIGTERM
    signal handlers as well.
    
    This does not scale if krb5info files are created for sub-domains as
    well. To remove the files automatically the removal is moved into a
    talloc destructor of an offline callback which is added if the file is
    created and frees itself when the system goes offline. Due to the
    talloc memory hierarchy we get removal on shutdown for free.
    
    Related to https://pagure.io/SSSD/sssd/issue/3652
    Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>