Add a STANDBY UNREGISTER command.

In some cases it is desirable to remove repmgr's handling of a standby, perhaps
because that standby is to be removed altogether.  With no UNREGISTER command,
one had to manually delete the corresponding row from repl_nodes.

README.md

@@ -350,6 +350,7 @@ Following event types currently exist:
 
     master_register
     standby_register
+    standby_unregister
     standby_clone
     standby_promote
     witness_create
@@ -458,6 +459,11 @@ its port if is different from the default one.
     promote/follow operations and to allow `repmgrd` to work with the node.
     An existing standby can be registered using this command.
 
+* `standby unregister`
+
+    Unregisters a standby with `repmgr`. This command does not affect the actual
+    replication.
+
 * `standby clone [node to be cloned]`
 
     Clones a new standby node from the data directory of the master (or