Resolution: Workflow is not loading the newer version of definition file.
Some times visual studio loads the older version of the workflow definition while debugging, even though you might have deleted an activity from the workflow, when you debug the code the definition file shows the older version where we the old activity will be still present.
Resolution
Visual studio loads the workflow definition from the dll which is available in GAC so in case we add/remove or do any modifications related to activities in the workflow, it is a good practice to uninstall the old dll from GAC and restart the VS so it can pick up the latest details.
Also you could clean up the browser history to avoid the any xml’s/ other files’ caching related issues.