lightnvm: remove debug lun statistics from gennvm

Since LUNs are managed internally on targets, the media manager has no
access to the free LUN lists. Thus, debug functions that show LUN
information on the device should not be implemented on the media
manager, but rather on the target in itself.

Signed-off-by: Javier González <javier@cnexlabs.com>
Signed-off-by: Matias Bjørling <m@bjorling.me>
Signed-off-by: Jens Axboe <axboe@fb.com>