Removing a FAILED node from repmgr cluster
As stated in https://groups.google.com/forum/#!topic/repmgr/R7osmnYLn4g :


  probably an UNREGISTER command is needed for that, for now... just
  delete the entry from repl_nodes 


This is done by accessing the master node with the same connection details used by repmgr and issuing the following:

DELETE FROM repmgr_&lt;cluster name&gt;.repl_nodes WHERE name = '&lt;name&gt;';