Remove DsInstance.request_service_keytab as it is not needed anymore
DsInstance.request_service_keytab() used to configure
/etc/sysconfig/dirsrv which is not needed anymore with 389-ds-base
1.4.1.2. Thus, the method became indistinguishable from the parent and
can be removed completely.
Reviewed-By: Christian Heimes <cheimes@redhat.com>