Remove obsolete interface adding for ipv6

eth6 doesn't exist, don't configure it at all,
instead of this the external interface wil be configured
according to network-template.yaml file settings.

Change-Id: Ib4ef9ee5a8ed04273ec6614c33f953642ac22d7c

roles/overcloud-prep-network/templates/overcloud-prep-network.sh.j2

@@ -75,20 +75,6 @@ EOF'
 
 sudo ifup ifcfg-{{ network.device_name }}
 
-{% else %}
-
-sudo bash -c 'cat <<EOF > /etc/sysconfig/network-scripts/ifcfg-eth6
-DEVICE=eth6
-ONBOOT=yes
-BOOTPROTO=static
-IPADDR={{ network.address }}
-NETMASK={{ network.netmask }}
-IPV6ADDR={{ network.address6 }}
-IPV6INIT=yes
-EOF'
-
-sudo ifup ifcfg-eth6
-
 {%endif%}