Removes a vertex from the 'assigned' list of vertices and from the given face
function(vertex, face) {
      if (vertex === face.outside) {
        // fix face.outside link

        if (vertex.next !== null && vertex.next.face === face) {
          // face has at least 2 outside vertices, move the 'outside' reference

          face.outside = vertex.next;
        } else {
          // vertex was the only outside vertex that face had

          face.outside = null;
        }
      }

      this.assigned.remove(vertex);

      return this;
    }