Use nis-domainname.service on all RH platforms
RHEL 8 and Fedora >= 29 use "nis-domainname.service" as service name for
domainname service. Remove special code in ipaplatform.rhel and for Fedora
< 28. Only Fedora 29+ is supported by IPA 4.8.
Fixes: https://pagure.io/freeipa/issue/8004
Signed-off-by: Christian Heimes <cheimes@redhat.com>
Reviewed-By: Rob Crittenden <rcritten@redhat.com>