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 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 are the Secretary, to free any elected role, you will have to click on the role to be removed, in the appropriate circle, and then click the remove button, as shown in screenshots below:

You will have a warning message pop-up. Click "Ok" to proceed if the person resigned from the role.

If you want to know how to re-assign an elected role, please check the article: 
How do I assign an elected role?