FALLUJAH, Iraq — This restive Sunni town, the first to rebel against U.S. troops a decade ago, is rising up once again, this time against the government the Americans left behind. In an echo of the Arab Spring protests that have overturned regimes elsewhere in the region, angry residents have been staging weekly demonstrations against the Shiite-led government of Prime Minister Nouri al-Maliki, calling, among a host of other demands, for him to step down and for Iraq’s U.S.-brokered constitution to be replaced.