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-client.h

@@ -118,7 +118,6 @@ static struct option long_options[] =
 /* "standby clone" options */
 	{"copy-external-config-files", optional_argument, NULL, OPT_COPY_EXTERNAL_CONFIG_FILES},
 	{"fast-checkpoint", no_argument, NULL, 'c'},
-	{"wal-keep-segments", required_argument, NULL, 'w'},
 	{"no-upstream-connection", no_argument, NULL, OPT_NO_UPSTREAM_CONNECTION},
 	{"recovery-min-apply-delay", required_argument, NULL, OPT_RECOVERY_MIN_APPLY_DELAY},
 	{"replication-user", required_argument, NULL, OPT_REPLICATION_USER},