Good question.
The user should be logged to remove a device.
Thus he has been authenticated either by the device he wants to remove or by another mean.
If he has been authenticated by another mean, he might wants to remove the device because it's been lost or malfunctioning, thus he can't get a code from it.
If he has been authenticated by the device he wants to remove, then one could ask : is it possible to make a mandatory rule stating "their should at least one OTP device on each account".
--
Ionel GARDAIS
Tech'Advantage CIO - IT Team manager