MDEV-19210: drop support for instantiated services from galera_new_cluster

Support in service file was dropped in 91f16948
(systemd: multi-instance not for Galera, User/Group flexible).

scripts/galera_new_cluster.sh

@@ -22,7 +22,7 @@ EOF
 fi
 
 echo _WSREP_NEW_CLUSTER='--wsrep-new-cluster' > @mysqlunixdir@/"wsrep-new-cluster" && \
-    systemctl restart ${1:-mariadb}
+    systemctl restart mariadb.service
 
 extcode=$?