lightnvm: remove unused lists from struct rrpc_block

The ->list, ->open_list, and ->closed_list lists were previously used
for statistics. However, their usage have been removed, and thus these
can safely be removed.

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