Remove an unused site from list

Remove an unused site from list

We have a Site listed that we would like to remove.
 

When I try to delete it, I get "FAILURE :OLDSITE Site is being used by a module. Hence cannot delete it."

Is there any way to determine where it is being used and remove that dependency. Or, is there a way to hide the Site from the dropdown menus so that it is no longer seen by the Technicians?
 
 

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