s390/dasd: remove unneeded sanity check

Reported by smatch that the usage of cqr->block is inconsistent.
The sanity check is not needed because _dasd_requeue_request already
checks for a valid cqr->block pointer and all referenced ERP requests
have a valid cqr->block pointer as well since it is copied during ERP
process.

Signed-off-by: Stefan Haberland <sth@linux.vnet.ibm.com>
Reviewed-by: Jan Hoeppner <hoeppner@linux.vnet.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>