sysdb: add sysdb_subdomain_content_delete()
sysdb_subdomain_content_delete() will remove all user and group objects
from a sub-domain container but not the sub-domain object and the user
and group container itself.
Related to https://pagure.io/SSSD/sssd/issue/4078
Reviewed-by: Pavel Březina <pbrezina@redhat.com>