default void preRemoveServers(final ObserverContext<MasterCoprocessorEnvironment> ctx,
    Set<Address> servers) throws IOException {
  }
Called before servers are removed from rsgroup
@param ctx     the environment to interact with the framework and master
@param servers set of decommissioned servers to remove
hbase-server/src/main/java/org/apache/hadoop/hbase/coprocessor/MasterObserver.java