Oh, I see. All along I thought you were asking for something different, to remove the link button altogether, and just have the [...] in all cases.
Removing it entirely only when the full post content has been displayed requires modifying the template code, because a test has to be set up to check if the displayed text ended in [...] and only display the button if it did.
I want to point out the reason you are having this issue is because you are using a test post that is one line long. Wouldn't it be unusual for a real post to be less than 55 words long? Anything over 55 words displays with the [...] and thus needs a button.
If you must hide the button only on short posts, that will require a template modification.