Nigerian villagers who are suing Chevron Corp. in a San Francisco federal court have quietly moved to withdraw half of their claims that the oil company was responsible for military attacks on protesters in the late 1990s. In papers filed last week, plaintiffs' lawyers, without explanation, asked a federal judge to dismiss claims by 25 Nigerians over a January 1999 attack on villages near oil facilities in the Niger Delta where residents had protested pollution. In their lawsuit, the plaintiffs had said government troops, using a helicopter and boats supplied by Chevron, killed at least four unarmed people and burned two villages to the ground. The dismissal does not affect the rest of the case, which is scheduled for trial in September... Chevron...denies any role in the Nigeria attacks and challenges the plaintiffs' description of themselves as peaceful demonstrators...[saying the] incident...was "a violent occupation of private property by aggressors seeking to extort cash payments from the company"...