BEIJING — China’s ruling Communist Party announced Thursday that it will abolish the country’s decades-old one-child policy and allow all couples to have two children, removing remaining restrictions that limited many urban couples to only one, the official Xinhua News Agency said. Xinhua said on its microblog that the decision to allow all couples to have two children was “to improve the balanced development of population” and to deal with an aging population. The decision does away with an unpopular policy that was long considered one of the party’s most onerous intrusions into family life. The decision was contained in a Communist Party communique that followed a meeting of the party’s Central Committee on planning the country’s economic and social development through 2020.