To remove a link, select the linked text and then click the "Insert link" icon, which will now have the title text "Modify link" (Img. 4.5.1).
The tab on the left will say "Remove link". Click the tab to remove the link (Img. 4.5.2).
Note that, when removing a link, it is important to not just delete the linked text. The link might remain in the HTML-code visible on the frontend, but not in the text editor.