sysdev: remove global sysdev drivers list

No one uses sysdev_drivers. Because no one calls sysdev_driver_register
with NULL class.

And it is difficult to imagine that someone want to implement a global
sysdev driver which is called with all sys_device on any kind of
sysdev_class.

So this patch removes global sysdev_drivers list and update comments
for this change.

Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
Cc: Tejun Heo <htejun@gmail.com>
Acked-by: Cornelia Huck <cornelia.huck@de.ibm.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>