There are three roles that are not assigned. Rather, they are elected. Those roles are Secretary, Facilitator, and Rep Link.
If someone resigns from their elected role and no elections are planned, the Secretary of the circle concerned (or an Admin) can free the person from the role and leave it unassigned until the next election.
If you are the Secretary, on the circle's page, go to the Roles tab and click on the person icon with a x sign to free the core role in question, as shown in screenshots below:
You will have a warning message pop-up. Click "Ok" to proceed if the person resigned from the role.
Note: If you are not an Admin or the Secretary of the circle in question, you'll have to request the removal from someone in those roles.
If you want to know how to re-assign an elected role, please check the article: How do I assign an elected role?