mgr/vol: keep clone source info even after cloning is finished

mgr/vol: keep clone source info even after cloning is finished

Instead of removing the information regarding source of a cloned
subvolume from the .meta file after the cloning has finished, keep it as
it is as the user may find it useful.

Fixes: https://tracker.ceph.com/issues/71266
Signed-off-by: Rishabh Dave <ridave@redhat.com>
(cherry picked from commit bbacfdae1a2e7eb60f91b75852bcd1096b6e3c84)

Conflicts:
src/pybind/mgr/volumes/fs/async_cloner.py
- open_clone_subvol_pair_in_vol() is not present on Squid branch leading
  to this conflict.