default void postBalanceRSGroup(final ObserverContext<MasterCoprocessorEnvironment> ctx,
    String groupName, BalanceRequest request, BalanceResponse response) throws IOException {
  }
Called after a region server group is removed
@param ctx       the environment to interact with the framework and master
@param groupName group name
@param request   the request sent to the balancer
@param response  the response returned by the balancer
hbase-server/src/main/java/org/apache/hadoop/hbase/coprocessor/MasterObserver.java