mgr/volumes: remove incorrect 'size' from output of 'snapshot info'

mgr/volumes: remove incorrect 'size' from output of 'snapshot info'

The 'size' shown in the output of snapshot info command relies on
rstats which is incorrect snapshot size. It tracks size of the
subvolume from the snapshot has been taken instead of the snapshot
itself. Hence having the 'size' field in the output of 'snapshot info'
doesn't make sense until the rstats is fixed.

Fixes: https://tracker.ceph.com/issues/55822
Signed-off-by: Nikhilkumar Shelke <nshelke@redhat.com>
(cherry picked from commit 9f514a0bd14b30f6ef6a77541f4cf75541b52322)