You mean the link that you see attached to the item in the center pane? You can just select it and delete as you would anything else in Zotero (right-click move to trash or press the Delete key).
My question would be why you're trying to do that. Keep in mind that the link does not count towards your zotero.org storage quota.
Upgrade Storage
My question would be why you're trying to do that. Keep in mind that the link does not count towards your zotero.org storage quota.