[SCSI] zfcp: remove unused device_unregister wrapper

Remove the now unused function zfcp_device_unregister since all
users have been converted to use device_unregister directly.

Reviewed-by: Steffen Maier <maier@linux.vnet.ibm.com>
Signed-off-by: Sebastian Ott <sebott@linux.vnet.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>
Signed-off-by: Steffen Maier <maier@linux.vnet.ibm.com>
Signed-off-by: James Bottomley <JBottomley@Parallels.com>