s390/cio: remove unused channel_subsystem_reinit

Added with commit 77e844b9 ("s390/hibernate: add early resume
function") unused since commit 39421627 ("s390: remove broken
hibernate / power management support").

Reviewed-by: Vineeth Vijayan <vneethv@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>