osd/PeeringState: do not complain about past_intervals constrained by oldest epoch

osd/PeeringState: do not complain about past_intervals constrained by oldest epoch

The start of the required interval has a floor set by the oldest osdmap
epoch we have.  That can lead to an invalid/empty required interval
(because the start is >= the end), but the PG may still have past
intervals.  That can be cause by a slow PG deletion.

No need to complain about this harmless condition.

Fixes: https://tracker.ceph.com/issues/39546
Signed-off-by: Sage Weil <sage@redhat.com>