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>