Remove m.new_device support
We now rely on the server to track new devices, and tell us about them when
users add them, rather than forcing devices to announce themselves (see
https://github.com/vector-im/riot-web/issues/2305 for the whole backstory
there).
The necessary support for that has now been in all the clients and the server
for several months (since March or so). I now want to get rid of the
localstorage store, which this code is relying on, so now seems like a good
time to get rid of it. Yay.
