Seems to be a deliberate change to the build specification.
The user manual for cars produced before 14th October 2015 shows a USB and AUX socket in the glove box, after that date the corresponding manual for UK cars shows only the AUX socket.
Yep, save Citroen 50c in manufacturing I guess. To be fair, the one in front of the gear stick does the same job and then we have a double-USB outlet plugged in to the 'lighter' socket for charging devices. Haven't missed having one in the glove box or the now defunct 16gb jukebox either.
To be fair I think the primary reason for the glove box USB was for the Citroen Connect Dongle (all the images in the book show it mounted in the dash connector). I would assume take up of that was pretty low and hence deemed surplus to requirements.