I found that watch faces created with Connect IQ's "Face It" cannot be uninstalled from Connect IQ or from the watch. I think the problem is that Connect IQ does not detect that it has installed the watch face, even though it has been installed. The "Install" button continues to say "Install" even after the watch face has been installed. So it cannot be uninstalled from the Face It screen.
Those watch faces also then do not appear on the device's Watch Faces screen in Connect IQ, so it can't be uninstalled either.
Work-around:
After installing a Face It watch face, I discovered that in the Connect app, if you tap on the device and go to Appearance, Watch Faces, these Face It watch faces will appear there in the "Not Installed" section at the bottom. If you tap on it there, Install it from that screen, then you are able to Uninstall it and it is removed from the watch.
So my guess is that the bug is in the Face It feature, not detecting that it has successfully installed the Watch Face.