Delta Air Lines said on Wednesday it will stop blocking middle seats as of May 1, a move that will allow it to start selling more tickets on each flight at a time when travel demand is picking up as more Americans receive COVID-19 vaccines. Atlanta-based Delta is now the only U.S. airline still limiting seat capacity on all the cabins of its planes to give passengers more space during the pandemic, though studies have shown that the risk of COVID-19 transmission in flight is low if everyone wears a mask. Delta’s decision to lift the middle seat block came alongside a series of other announcements, including the resumption of onboard snack and beverage service starting April 14. The airline is also extending the validity of tickets expiring in 2021, and those bought this year, through the end of 2022, it said. Shares of Delta were down 1.4 percent in …