repmgrd: restrict BDR monitoring to two node setup

It's not safe to have more than two nodes with this kind of
"failover", so we don't need to select alternative nodes by
priority.

repmgrd.c

@@ -37,7 +37,6 @@ PGconn	   *local_conn = NULL;
 /* Collate command line errors here for friendlier reporting */
 static ItemList	cli_errors = { NULL, NULL };
 
-int			server_version_num = 0;
 bool        startup_event_logged = false;
 
 MonitoringState monitoring_state = MS_NORMAL;
@@ -57,7 +56,6 @@ static void show_usage(void);
 static void daemonize_process(void);
 static void check_and_create_pid_file(const char *pid_file);
 
-
 static void start_monitoring(void);