Fixes ISSUE-48221: Remove 2.50 bits of Linked Items code
The UsedByLink servlet still had unused 2.50 era code inside which is no longer used. Delete those unused part:
- Commands DEFAULT and links
- the implemented and xmlEngine templates related to that
- the url mapping for UsedByLink_Detail.html (which is unused)
- the openLinkedItems function of the ClassicOBCompatibility class as it was only referenced from what is removed here
Try: https://builds.openbravo.com/view/try/job/try-init/1115/