Hello
THis is a standard feature of SAP. IF the workcenter is no more in use and you do not want to use this further in any of the new routing than please follow the path.
Go to change work center CO02 ->in basic data tab - > go to usage - put there "000 - no task list categories".
With this settings you can not use this furhter in any of the routings.
OR
You may mark delete the work center - CR02 - Tab work center - Delete.
Hope this will help
Regards
SS