null_blk: remove lightnvm support

With rrpc to be removed, the null_blk lightnvm support is no longer
functional. Remove the lightnvm implementation and maybe add it to
another module in the future if someone takes on the challenge.

Signed-off-by: Matias Bjørling <m@bjorling.me>
Signed-off-by: Jens Axboe <axboe@kernel.dk>