scsi: libsas: remove unused port_gone_completion and DISCE_PORT_GONE

No one uses the port_gone_completion in struct asd_sas_port and
DISCE_PORT_GONE in enum disover_event, clean them out.

Signed-off-by: Jason Yan <yanaijie@huawei.com>
CC: Johannes Thumshirn <jthumshirn@suse.de>
CC: Ewan Milne <emilne@redhat.com>
CC: Christoph Hellwig <hch@lst.de>
CC: Tomas Henzl <thenzl@redhat.com>
CC: Dan Williams <dan.j.williams@intel.com>
Reviewed-by: Johannes Thumshirn <jthumshirn@suse.de>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>