Within your Assignments tab in a classroom, you will have a list of all assignments.
To see all of your assignments ever created for the given class, select the All bucket and ensure your filter is set to Due: All-time.
In the list for each assignment, there will be an Options button. Click the button, and select Delete.
Deleting an assignment will delete all of the data in the assignment, and it will be removed from the assignment lists for both teachers and students.