I have the same question and it is appalling that nobody from Adobe has replied; I was "chatting" with an advisor (I had asked the same question) and they explained that chat support isn't available for Digital Edition but if you post your Q here they will respond. I can see that isn't the case at all.
click edit . click the highlighter pen. put pointer on the highlighted area. click. a box should appear around the highlighted area. hit DELETE. Voila! done. (today is 04/13/18)