OKLAHOMA CITY — The former girlfriend of a man accused of killing four people, including a prostitute featured on HBO’s “Cathouse” series, testified Tuesday she overheard him plan the Oklahoma City shootings because he was unhappy with the amount of money he made from a drug dealer among those killed.Denny Edward Phillips, 34, is charged in the deaths of drug dealer Casey Mark Barrientos and three others, including two pregnant women. One of the victims was Brooke Phillips, who had worked for a legal Nevada brothel featured on HBO. She was not related to Denny Phillips.In testimony at Denny Phillips’ …