Thank you for this post. It is a great description on how to remove a workflow association from a list for workflows for SharePoint online, but this does not delete the workflow.
After removing the association from the list, you can click Add a Workflow, then choose the same workflow from the lists of workflow templates.
Currently, Nintex for O365 does not have a simple method to delete the workflow in its entirety. But this can be done with powershell if necessary.