和关注反着来就行
根据writerId和粉丝id查询用户
执行删除操作
redis粉丝数 - 1

    public void deleteFans(String writerId, String fanId) {
        QueryWrapper<Fans> queryWrapper = new QueryWrapper<>();
        queryWrapper.eq("writer_id", writerId);
        queryWrapper.eq("fan_id", fanId);

        fanMapper.delete(queryWrapper);

        String redisWriterFollowCounts = redisOperator.get(REDIS_WRITER_FOLLOW_COUNTS + ":" + writerId);
        String redisMyFollowCounts = redisOperator.get(REDIS_MY_FOLLOW_COUNTS + ":" + writerId);

        if (Integer.parseInt(redisWriterFollowCounts) > 0) {
            redisOperator.decrement(REDIS_WRITER_FOLLOW_COUNTS + ":" + writerId, 1);
        }

        if (Integer.parseInt(redisMyFollowCounts) > 0) {
            redisOperator.decrement(REDIS_MY_FOLLOW_COUNTS + ":" + fanId, 1);
        }

    }