s390/cio: remove invalid condition on IO_SCH_UNREG

The condition to check the cdev pointer validity on
css_sch_device_unregister() is a leftover from the 'commit 8cc0dcfd
("s390/cio: remove pm support from ccw bus driver")'. This could lead to a
situation, where detaching the device is not happening completely. Remove
this invalid condition in the IO_SCH_UNREG case.

Link: https://lore.kernel.org/r/20210423100843.2230969-1-vneethv@linux.ibm.com
Fixes: 8cc0dcfd

 ("s390/cio: remove pm support from ccw bus driver")
Reported-by: Christian Ehrhardt <christian.ehrhardt@canonical.com>
Suggested-by: Christian Ehrhardt <christian.ehrhardt@canonical.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Vineeth Vijayan <vneethv@linux.ibm.com>
Tested-by: Julian Wiedmann <jwi@linux.ibm.com>
Reviewed-by: Peter Oberparleiter <oberpar@linux.ibm.com>
Tested-by: Christian Ehrhardt <christian.ehrhardt@canonical.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>