Source

File: wp-includes/class-wp-roles.php

public function remove_role( $role ) {
		if ( ! isset( $this->role_objects[ $role ] ) ) {
			return;
		}

		unset( $this->role_objects[ $role ] );
		unset( $this->role_names[ $role ] );
		unset( $this->roles[ $role ] );

		if ( $this->use_db ) {
			update_option( $this->role_key, $this->roles );
		}

		if ( get_option( 'default_role' ) == $role ) {
			update_option( 'default_role', 'subscriber' );
		}
	}