Chosen Solution
I have two Harmony remotes paired to one hub, with one of those remotes paired to another hub at the same time. However, I would like to remove that one remote from the former hub, so that it’s not sending the same commands to both hubs at once.
Device management can be done from the Harmony app. In the app, tap Menu > Settings> Edit Devices and Activities> Devices. Select the device you want to delete, tap Delete, and then confirm the decision on the next screen.