Deletes a vote for a poll.

def retract(args):
    """Deletes a vote for a poll."""
    if not args.msg:
        return "Syntax: !vote retract <pollnum>"
    if not args.msg.isdigit():
        return "Not A Valid Positive Integer."
    response = get_response(args.session, args.msg, args.nick)
    if response is None:
        return "You haven't voted on that poll yet!"
    args.session.delete(response)
    return "Vote retracted"