TechCrunch is dropping online ad network RadiumOne as a sponsor of the popular Disrupt NY technology conference over charges that its CEO beat his girlfriend during a horrific episode of domestic violence. In an open letter to RadiumOnes board members that was posted online Saturday, Leena Rao, a senior editor at TechCrunch, explained that the behavior of RadiumOne CEO Gurbaksh Chahal had become impossible to ignore. …