scsi: remove superfluous NULL pointer check from scsi_kill_request()

Dan's list included:

drivers/scsi/scsi_lib.c +1365 scsi_kill_request(9) warning: variable derefenced in initializer 'cmd'
drivers/scsi/scsi_lib.c +1365 scsi_kill_request(9) warning: variable derefenced before check 'cmd'

We dereference cmd (and possible OOPS if cmd == NULL) before starting the
request so just remove the superfluous debugging code altogether.

[ bart: the potential NULL pointer dereference was finally fixed in
  (much later than mine) commit 03b14708 but my patch is still valid ]

Reported-by: Dan Carpenter <error27@gmail.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Eugene Teo <eteo@redhat.com>
Signed-off-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>
Cc: James Bottomley <James.Bottomley@HansenPartnership.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>