Project > List
With the project in context, click the X on the right-hand side of the role to delete and click Remove to confirm.
Project > Gantt
With View Project Roles enabled in the Gantt settings, click on the pencil icon next to the role you wish to delete. Click on the Remove Role button in the role window and click Remove to confirm.
Note: All history for the role and associated people will be lost once the role is deleted. This cannot be undone.