Protesters campaigning for the withdrawal of the controversial Hong Kong extradition bill returned to MTR stations on Friday, dressed in black and urging passengers to disrupt services, students to skip classes and office workers to go on strike.After causing delays on the MTR on Thursday, demonstrators called for services to be disrupted at Kowloon Tong and Central stations, while hundreds of people dressed in black appeared in a silent protest at Kowloon Tong.Seven members of pro-democracy…