cls_rgw: only maintain object if there are preserved xattrs

rgw.obj_remove op doesn't need to keep the object around if there aren't
preserved xattrs found.

Signed-off-by: Yehuda Sadeh <yehuda@redhat.com>
rgw: no need to set tag on object we're removing

Signed-off-by: Yehuda Sadeh <yehuda@redhat.com>
rgw: only set olh attr on actual olh object

Signed-off-by: Yehuda Sadeh <yehuda@redhat.com>