scsi_lib: remove unused variable

The last request completion cleanup in scsi_lib left an unused
this_count variable in scsi_io_completion().
(It was used before in a code segment that now uses blk_end_request_all())

Signed-off-by: Boaz Harrosh <bharrosh@panasas.com>
Acked-by: Tejun Heo <tj@kernel.org>
Acked-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>