device: remove the prefix-delegation IP configuration on cleanup

When a device in IPv6 shared mode obtains a prefix, it adds a new l3cd
of type L3_CONFIG_DATA_TYPE_PD_6 for that prefix. However, that l3cd
is never removed later and so the address lingers on the interface
even after the connection goes down. Remove the l3cd on cleanup.

src/core/devices/nm-device.c

@@ -13457,6 +13457,8 @@ _dev_ipsharedx_cleanup(NMDevice *self, int addr_family)
         nm_clear_l3cd(&priv->ipshared_data_4.v4.l3cd);
 
         _dev_l3_register_l3cds_set_one(self, L3_CONFIG_DATA_TYPE_SHARED_4, NULL, FALSE);
+    } else {
+        _dev_l3_register_l3cds_set_one(self, L3_CONFIG_DATA_TYPE_PD_6, NULL, FALSE);
     }
 
     _dev_ipsharedx_set_state(self, addr_family, NM_DEVICE_IP_STATE_NONE);