Good advice. I reset it again just now, and the extra "Reminders" list is gone from the watch.
In summary: yesterday I unpaired the watch (so that it would back-up), reset it, and restored from backup. Extra list still there. Then I reset the watch (on the watch itself, in settings), and restored as new. Extra list still there. Then this morning (following advice immediately above) I reset from the watch again, and set up as new watch. Extra list gone
One thing during this morning's "reset and new" that I don't remember seeing yesterday was a screen that asked me whether I wanted to return to factory defaults. But I might have seen it yesterday and forgotten. In any case, I said "yes".
For those nervous about doing a "setup as new" rather than "restore from backup" --- the only thing I lost was a customised watch-face and a downloaded playlist. Most stuff (so far) is not stored on the watch itself. In particular it's not like resetting your iPhone to factory defaults, and then re-customising from memory: that takes ages.
Thanks bgrace for the advice.