cmake: remove device_extern.h logic

This file is no longer used by device.h, so let's avoid spending time
generating the content formerly in it.

In order to leave a pointer in place for users who are expecting to
see it or are pulling it into their own builds, however, replace its
contents with an #error directive that tells them what happened. This
can be removed later on when we expect people are used to the
transition.

Signed-off-by: Martí Bolívar <marti.bolivar@nordicsemi.no>