TOKYO — Japan's self-imposed nuclear freeze has ended, less than two months after every one of the country's reactors were shut down in the wake of the 2011 Fukushima Daiichi disaster. As expected, the prime minister, Yoshihiko Noda, on Saturday approved the immediate restart of two reactors at Oi nuclear power plant under intense pressure for businesses concerned about summer power cuts and disruption to manufacturers. Japan's last working reactor was turned off in early May, leaving it without nuclear power for the first time in more than 40 years.