Remove IPv6/active/preference fields from eNBs.[0].mme_ip_address array

In a previous commit, the functionality of handling IPv6 (and DNS names)
as part of the IPv4 was introduced (in IPv4 field for backwards
compatibility). This commit now removes superfluous configuration
fields:
- IPv6:  handled by IPv4 field
- active: users can just remove an entry
- preference: not needed

ci-scripts/conf_files/enb.nsa.band7.25prb.usrpb200.conf

@@ -177,13 +177,7 @@ eNBs =
     enable_measurement_reports = "yes";
 
     ////////// MME parameters:
-    mme_ip_address      = ( { ipv4       = "172.21.16.137";
-                              ipv6       = "192:168:30::17";
-                              port       = 36412 ;
-                              active     = "yes";
-                              preference = "ipv4";
-                            }
-                          );
+    mme_ip_address = ({ ipv4 = "172.21.16.137"; port = 36412; });
 
     ///X2
     enable_x2         = "yes";