mgr/volumes: Remove stale snapshot user metadata

mgr/volumes: Remove stale snapshot user metadata

This patch adds the capability to remove the stale snapshot user
metadata while loading the subvolume if it is present. It can't
be done in 'SubvolumeBase.discover' since v1 and v2 snapshot paths
are different. This is done just after the discover before returning
the specific version object.

Fixes: https://tracker.ceph.com/issues/55976
Signed-off-by: Kotresh HR <khiremat@redhat.com>
(cherry picked from commit 65af2d123a1f1ef9c4b370e908ece588eec19a1f)