mgr/rbd_support: remove CreateSnapshotRequests __del__()

mgr/rbd_support: remove CreateSnapshotRequests __del__()

There is no need for CreateSnapshotRequests.__del__() that calls
CreateSnapshotRequests.wait_for_pending().
MirrorSnapshotScheduleHandler.shutdown() already calls
CreateSnapshotRequests.wait_for_pending().

Signed-off-by: Ramana Raja <rraja@redhat.com>
(cherry picked from commit fed1e87685a698876cf167b3681327e5b0066ee6)

Conflicts:
       src/pybind/mgr/rbd_support/mirror_snapshot_schedule.py
 - Above conflict was due to commit e4a16e2
   ("mgr/rbd_support: add type annotation") not in pacific