dasd: remove dead code

Since e443343e we haven't had a request_fn attached to
this driver, hence any code inside an if (q->request_fn) is
unreachable.

Fixes: e443343e

 ("s390/dasd: blk-mq conversion")
[sth: Keep and fix the dasd_info->chanq_len counter.]
Reviewed-by: Hannes Reinecke <hare@suse.com>
Reviewed-by: Jan Hoeppner <hoeppner@linux.ibm.com>
Signed-off-by: Stefan Haberland <sth@linux.ibm.com>
Tested-by: Ming Lei <ming.lei@redhat.com>
Reviewed-by: Omar Sandoval <osandov@fb.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>