By Michael Noble Jr. TULSA, Okla. (Reuters) -A gunman who fatally shot five people including himself at an Oklahoma medical building after buying an assault-style rifle the same day had gone there to kill a doctor who he blamed for back pain he felt after surgery, authorities said on Thursday. The suspect entered a building on Tulsa’s St. Francis Health System campus with a semi-automatic weapon on Wednesday and opened fire at anyone he encountered, Tulsa Police Chief Wendell Franklin said during a news briefing. Two doctors and two other people were killed. The suspect “came in with the inten…