scsi_debug: resp_request: remove unused variable

Fixes the following warning

In function ‘resp_requests’:
drivers/scsi//scsi_debug.c:1432:15: warning: variable ‘want_dsense’ set
but not used [-Wunused-but-set-variable]
  bool dsense, want_dsense;

Signed-off-by: Tomas Winkler <tomas.winkler@intel.com>
Reviewed-by: Martin K. Petersen <martin.petersen@oracle.com>
Signed-off-by: James Bottomley <JBottomley@Odin.com>