temp fix for lumi report on workloadsummary

temp fix for #5216 
We can remove the line
https://github.com/ticoann/WMCore/blob/temp_fix_lumi_update_ws/src/python/WMComponent/TaskArchiver/TaskArchiverPoller.py#L720
Since I don't see any duplicate in the db when I looked at it. However I will be more comfortable to be safe.
I am not sure how much performance will increase by switching to set right there instead further outside the loop. But I guess better than the current one. I will add the code and create the new patch. Thanks