Delete Links Between Cards from the Links Tab
The Links tab of the card not only helps users to create new links and associate already existing cards but also allows them to easily delete links.
To unlink cards -> open up a card and go to the Links tab.
Use the Delete links button, to activate a panel and select/checkmark the cards' links you want to delete.
In case that a user wants to unlink a particular card, just right click on its representation in the Links tab panel and use the Delete links function.
Note: this action will not delete the cards themselves -> it will delete the links between them only.