Remove option "--wal-keep-segments"

This is a remnant of the early repmgr days when there were no alternative
mechanisms for ensuring sufficient WAL remains available while cloning a
standby.

The purpose of this setting was to override a check for an (arbitrary)
minimum setting for "wal_keep_segments". As there's no reliable way
of determining a sensible value for this, and improvements in
pg_basebackup mean WALs can be streamed (possibly using a replication
slot) while the backup is in progress, there's no point in keeping
this around.

We will however still emit a warning about setting "wal_keep_segments"
if the configuration doesn't appear to provide any other way of
ensuring WAL is available during/after the cloning process and
"wal_keep_segments" is not set.

repmgr-action-standby.c

@@ -87,10 +87,22 @@ static CheckStatus parse_node_check_archiver(const char *node_check_output, int
 static CheckStatus parse_node_check_replication_lag(const char *node_check_output, int *seconds, int *threshold);
 
 /*
- * do_standby_clone()
+ * STANDBY CLONE
  *
  * Event(s):
  *  - standby_clone
+ *
+ * Parameters:
+ *  --upstream-conninfo
+ *  --no-upstream-connection
+ *  -F/--force
+ *  --dry-run
+ *  -c/--fast-checkpoint
+ *  --copy-external-config-files
+ *  --recovery-min-apply-delay
+ *  --replication-user XXX only required if no upstream record
+ *  --use-recovery-conninfo-password
+ *  --without-barman
  */
 
 void