Remove "at DL1" from ipa-server-install man page
As there is currently only DL1, there is no need to have extra
sentences for "at domain level 1".
See: https://pagure.io/freeipa/issue/7669
Signed-off-by: Thomas Woerner <twoerner@redhat.com>
Reviewed-By: Christian Heimes <cheimes@redhat.com>