The Short Answer:

Since a tweet has an action menu in the upper-right corner, with a "Delete" option, I expected to find the same for retweets. Instead, the action menu for a retweet on my own profile has actions for the original tweet and the account that posted it. I had followed up my original retweet with another retweet, plus comment, and I didn't want both retweets on my profile. To undo my original retweet, all I had to do was tap the lit-up retweet icon and choose "Undo Retweet", as described above.