stellio: drop IPv6 workarounds now upstream in cnx-network

Both interim blocks are in cnx-network b2b5daa:

- 27c697a adds vlans.<name>.ipv6SubnetId (lan -> 0, others -> VLAN id),
  replacing the SubnetId=auto race the mkForce pins worked around. mgmt
  moves from 0x1 to 0xA; on AIS's bare /64 both are skipped alike.
- 45643d6 is the router-radv-deprecate unit this file carried a copy
  of. Keeping both concatenated the two scripts into one unit running
  two ip-monitor loops.

machines/stellio/configuration.nix

@@ -84,97 +84,6 @@
       22
     ];
 
-    # AIS delegates a single /64 over PPPoE (seen 2026-10-02: "ppp0: DHCP:
-    # received delegated prefix 2405:9800:b910:1f5d::/64"). The router
-    # service hands out sub-prefixes with SubnetId=auto, so whichever VLAN
-    # interface comes up first takes the whole /64 -- on this box vlan-mgmt
-    # did, and vlan-lan logged "Couldn't find a suitable prefix. Ran out of
-    # address space": no RA prefix on lan, so clients had no global IPv6.
-    # Pin the ids: lan (where the clients are) always gets subnet 0, i.e.
-    # the entire prefix when it is a /64; mgmt asks for subnet 1, which
-    # fails harmlessly on a /64 (mgmt is reached over v4 and the meshes) and
-    # starts working unchanged if AIS ever delegates wider. Drop once
-    # cnx-network's router service exposes a per-VLAN subnet id.
-    systemd.network.networks."40-vlan-lan".dhcpPrefixDelegationConfig.SubnetId = lib.mkForce "0x0";
-    systemd.network.networks."40-vlan-mgmt".dhcpPrefixDelegationConfig.SubnetId = lib.mkForce "0x1";
-
-    # Interim copy of cnx-network-clan 45643d6 (router/ipv6: deprecate
-    # withdrawn DHCPv6-PD prefixes); drop when the input is bumped past it.
-    # networkd never deprecates a prefix it stops announcing (systemd#29651):
-    # on every reboot the first PPPoE session lands on another AIS BNG, is
-    # torn down minutes later, and the reconnect brings a different /64.
-    # networkd announces the new prefix at once but clients keep the old
-    # one preferred for the rest of its 1800 s lifetime, so RFC 6724 source
-    # selection picks the dead prefix and every v6 connection fails for up
-    # to 30 minutes (2026-10-08 07:57). Watch vlan-* for networkd removing a
-    # dynamic (PD) address and send up to three RAs with that /64 at
-    # preferred lifetime 0, one short-lived radvd per RA (SIGKILLed: no
-    # shutdown RA with router lifetime 0), stopping as soon as the prefix is
-    # back on the interface -- a plain flap re-delegates the same /64 within
-    # seconds and a deprecating RA sent after networkd re-announced it would
-    # leave clients deprecated until the next periodic RA (08:19 today).
-    systemd.services.router-radv-deprecate = {
-      description = "Deprecate withdrawn DHCPv6-PD prefixes on the VLANs";
-      wantedBy = [ "multi-user.target" ];
-      after = [ "systemd-networkd.service" ];
-      path = [
-        pkgs.iproute2
-        pkgs.ipcalc
-        pkgs.radvd
-      ];
-      serviceConfig = {
-        Restart = "always";
-        RestartSec = 2;
-        RuntimeDirectory = "router-radv-deprecate";
-      };
-      script = ''
-        conf=$RUNTIME_DIRECTORY/radvd.conf
-        pid=$RUNTIME_DIRECTORY/radvd.pid
-
-        # /64s of the global addresses currently on an interface.
-        prefixes_on() {
-          ip -o -6 addr show dev "$1" scope global | while read -r _ _ _ a _; do
-            echo "$(ipcalc --no-decorate -n "$a")/''${a##*/}"
-          done
-        }
-
-        # "Deleted 4: vlan-lan    inet6 2001:db8::1/64 scope global dynamic mngtmpaddr \ valid_lft ..."
-        ip -o -6 monitor address | while read -r event _ dev _ addr _ scope rest; do
-          [[ $event == Deleted && $dev == vlan-* && $scope == global && $rest == *dynamic* ]] || continue
-          prefix="$(ipcalc --no-decorate -n "$addr")/''${addr##*/}"
-          cat > "$conf" <<EOF
-        interface $dev {
-          AdvSendAdvert on;
-          MinRtrAdvInterval 3;
-          MaxRtrAdvInterval 4;
-          AdvDefaultLifetime 1800;
-          AdvCurHopLimit 0;
-          prefix $prefix {
-            AdvOnLink on;
-            AdvAutonomous on;
-            AdvPreferredLifetime 0;
-            AdvValidLifetime 7200;
-          };
-        };
-        EOF
-          for _ in 1 2 3; do
-            if prefixes_on "$dev" | grep -qxF "$prefix"; then
-              echo "$dev: $prefix is back, leaving it announced"
-              break
-            fi
-            echo "$dev: deprecating withdrawn prefix $prefix"
-            rm -f "$pid"
-            # radvd sends its first RA on start; the next would follow 3-4 s later.
-            radvd -C "$conf" -p "$pid" -n -m stderr &
-            sleep 1
-            kill -KILL $! 2>/dev/null || true
-            wait $! || true
-            sleep 2
-          done
-        done
-      '';
-    };
-
     systemd.services.heartbeat-push = {
       description = "Heartbeat push to uptime monitor";
       serviceConfig = {