mon/MDSMonitor: fix check for standby reversion

mon/MDSMonitor: fix check for standby reversion

A standby-replay daemon always has a rank, so this check is completely
wrong. If a beacon from a standby-replay daemon reaches
MDSMonitor::prepare_beacon, it will always be evicted/removed by the
mons. This is rare (usually a reply occurs directly from
MDSMonitor::preprocess_beacon) but can happen in certain circumstances,
like a health warning included in the beacon.

Fixes: https://tracker.ceph.com/issues/56666
Signed-off-by: Patrick Donnelly <pdonnell@redhat.com>
(cherry picked from commit b2c40cc07faa3751e28e946bc10d9700621db140)