Bug 30016: Remove GetOpenIssue subroutine

Bug 30016: Remove GetOpenIssue subroutine

This patch adjusts the code that uses GetOpenIssue to use/find a Koha::Checkout object
instead

To test:
1 - Add a course to course reserves
2 - Create an item with barcode TESTKOC
3 - Add the item to a course
4 - Checkout the item
5 - View course details on stff and opac and confirm item shows as checked out and due date displays
6 - prove t/db_dependent/Circulation/issue.t t/db_dependent/Circulation.t t/db_dependent/CourseReserves.t
7 - Browse to Circulation->Upload offline circulation
8 - Upload a file to return the item: https://wiki.koha-community.org/wiki/Koha_offline_circulation_file_format
9 - Confirm item is returned

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>