librbd/mirror: remove peer UUID of group snapshot before attempting removal

librbd/mirror: remove peer UUID of group snapshot before attempting removal

Problem:
When the number of snapshots exceeds the configured limit
(rbd_mirroring_max_mirroring_snapshots), the cleanup process remove the last
group snapshot on the primary. If this removal fails partially, the
group snapshot with some or empty image snap may still remain.

As a result:
* The snapshot is incorrectly considered for synchronization to the secondary.
* The secondary snapshot can remain forever in the incomplete state, leading
  to stuck of daemon progress.

Solution:
Before attempting to remove the group snapshot:
* Explicitly remove the peer UUID associated with the snapshot.
* This ensures that the snapshot is no longer considered eligible for
  synchronization.
* Even if the actual snapshot deletion fails partially, the snapshot is not
  tried for mirroring, avoiding daemon stalls or inconsistencies.

Signed-off-by: Prasanna Kumar Kalever <prasanna.kalever@redhat.com>
Resolves: rhbz#2392398