scsi: tcmu: refactor rm_device cmd with new nl helpers

use new netlink events helpers tcmu_netlink_init() and
tcmu_netlink_send() to refactor netlink event TCMU_CMD_REMOVED_DEVICE

Signed-off-by: Zhu Lingshan <lszhu@suse.com>
Acked-by: Mike Christie <mchristi@redhat.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>