NVMe:Remove unreachable code in nvme_abort_req

Removing unreachable code from nvme_abort_req as nvme_submit_cmd has no
failure status to return.

Signed-off-by: Sunad Bhandary <sunad.s@samsung.com>
Acked-by: Keith Busch <keith.busch@intel.com>
Signed-off-by: Jens Axboe <axboe@fb.com>