null_blk: remove unused fields in 'nullb_cmd'

'list', 'll_list' and 'csd' are no longer used.

The 'list' is not used since it was introduced by commit f2298c04
("null_blk: multi queue aware block test driver").

The 'll_list' is no longer used since commit 3c395a96 ("null_blk: set a
separate timer for each command").

The 'csd' is no longer used since commit ce2c350b ("null_blk: use
blk_complete_request and blk_mq_complete_request").

Reviewed-by: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: Dongli Zhang <dongli.zhang@oracle.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>