selector-ready and order-ready line items are needlessly protected against deletion by other users

Steps to reproduce, in 3.1:
* As user 1, create a line item (I used Acquisitions > MARC Federated Search to do this). Check the box and do Actions > Mark Ready for Selector (or Order).
* As user 2, search for that line item (Acquisitions > General Search).
* As user 2, check the box and do Actions > Delete Selected Items.
* As user 2, repeat your search. Note that the line item wasn't actually deleted.

It makes sense to protect most line items against deletion by other users. But in this case, one of the possible actions the selector/orderer might take is to delete the item (because it's a duplicate, etc.) My understanding is that one of the uses of Mark Ready for Selector is to allow an approver to approve certain line items and deny others that they receive from various selectors. This bug -- especially when combined with https://bugs.launchpad.net/evergreen/+bug/1800745 -- really limits the abilities that the approver has to work with items that have been marked selector-ready.