PG: call check_recovery_sources in remove_down_peer_info

If we transition out of peering due to affected
prior set, we won't trigger start_peering_interval
and check_recovery_sources won't get called.  This
will leave an entry in missing_loc_sources without
a matching missing set.  We always want to
check_recovery_sources with remove_down_peer_info.

Fixes: 4805
Backport: bobtail
Signed-off-by: Samuel Just <sam.just@inktank.com>
Reviewed-by: Sage Weil <sage@inktank.com>