It’s a known issue that Whisperfish doesn’t currently show the device names properly. We’ll fix that, but I don’t think it’s going to be in the next release.
According to the screenshot, you’re trying to delete the first (primary) device, which is not possible. Signal Desktop seems to be the second device on the list, you should be able to remove that. The third device seems to be another Whisperfish.
We also should make the primary device and the current device more clear.