nvme: remove nvme_execute_passthru_rq

After moving the nvme_passthru_end call to the callers of
nvme_execute_passthru_rq, this function has become quite pointless,
so remove it.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Jens Axboe <axboe@kernel.dk>
Reviewed-by: Chaitanya Kulkarni <kch@nvidia.com>