Remove the temporary backport branch.

        Switch to the default branch before that.

def cleanup_branch(self, branch):
        """Remove the temporary backport branch.

        Switch to the default branch before that.
        """
        set_state(WORKFLOW_STATES.REMOVING_BACKPORT_BRANCH)
        self.checkout_default_branch()
        try:
            self.delete_branch(branch)
        except subprocess.CalledProcessError:
            click.echo(f"branch {branch} NOT deleted.")
            set_state(WORKFLOW_STATES.REMOVING_BACKPORT_BRANCH_FAILED)
        else:
            click.echo(f"branch {branch} has been deleted.")
            set_state(WORKFLOW_STATES.REMOVED_BACKPORT_BRANCH)