rgw: fix leak in RGWMetadataManager::remove()

Backport: dumpling
Fixes: #6445
handler->get() returns a newly allocated obj, need to delete it when
done.

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>
Merge pull request #664 from ceph/wip-6445

Wip 6445
Reviewed-by: Josh Durgin <josh.durgin@inktank.com>