rbd-mirror: avoid stuck group on secondary

rbd-mirror: avoid stuck group on secondary

The following steps leaves stale group on seondary left undeleted,
1. Create and mirror enable a group with 2 images.
2. Let it sync to the secondary
3. Demote on the primary and promote on the secondary
4. Wait until it starts replaying on the original primary
5. Delete the group on the new primary

Credits to Nithya Balachandran for highlighting the issue with detailed steps.

Signed-off-by: Prasanna Kumar Kalever <prasanna.kalever@redhat.com>