rgw: multi object delete should be idempotent

Fixes: #7346
When doing a multi object delete, if an object does not exist then we
should return a success code for that object.

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>
Merge pull request #1217 from ceph/wip-7346

rgw: multi object delete should be idempotent

Reviewed-by: Josh Durgin <josh.durgin@inktank.com>