Chinese e-commerce company Pinduoduo Inc.’s founder and chairman, Colin Huang, stepped down from the company on Wednesday, even as the five-year-old company overtook Alibaba Group Holding Ltd. to become the country’s largest e-commerce company by annual active buyers.Mr. Huang, 41, is resigning as China’s powerful internet sector comes under growing government scrutiny. His resignation follows another departure from a major company in the sector: Financial-tech giant Ant Group Co.’s Chief Executive Simon Hu stepped down earlier this month.