Remove address towers from a node registration.

     REMOVE [DECNET] NODE node-name
         TOWERS {tower-set}
         [ DIRECTORY_SERVICE dir-service ]
         [ PHASEIV_PREFIX addr-prefix ]
         [ REVERSE_DIRECTORY rev-dir-name ]

 This is used when an address tower is no longer available on a node, and
 the addressing information needs to be removed from the node registration
 in a directory service.