Remove unused DCB callback code

The highwater and lowwater callbacks were never registered for the client
DCBs in the binlogrouter.

The DCB hangup callbacks were never called by the core and were replaced
with fake hangup events in an earlier version.

server/modules/routing/binlogrouter/blr_slave.c

@@ -2289,9 +2289,6 @@ blr_slave_binlog_dump(ROUTER_INSTANCE *router, ROUTER_SLAVE *slave, GWBUF *queue
         blr_slave_read_ste(router, slave, fde_end_pos);
     }
 
-    slave->dcb->low_water  = router->low_water;
-    slave->dcb->high_water = router->high_water;
-
     dcb_add_callback(slave->dcb, DCB_REASON_DRAINED, blr_slave_callback, slave);
 
     slave->state = BLRS_DUMPING;