scsi: libsas: Remove useless dev_list delete in sas_ex_discover_end_dev()

The domain device 'child' is allocated in sas_ex_discover_end_dev() and
used to be added to the dev_list in this function. After the following two
fixes the device is added to the disco_list instead. As a result, the
list_del() and locking left behind is now redundant.

Fixes: 87c8331f ("[SCSI] libsas: prevent domain rediscovery competing with ata error handling")
Fixes: 92625f9b

 ("[SCSI] libsas: restore scan order")
Cc: John Garry <john.g.garry@oracle.com>
Signed-off-by: Jason Yan <yanaijie@huawei.com>
Reviewed-by: John Garry <john.g.garry@oracle.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>