lightnvm: remove get_lun operation on gennvm

Since LUNs are managed internally on the target, there is no need for
the media manager to implement a get_lun operation.

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