A federal judge has rejected an effort by UnitedHealth Group Inc. to bar a former employee from joining a new health care venture by Amazon.com Inc. (Nasdaq: AMZN), Berkshire Hathaway Inc. (NYSE: BRK.B) and JP Morgan Chase & Co. (NYSE: JPM). Reuters has a report on the decision by U.S. District Judge Mark Wolf in Boston, who refused to issue a temporary restraining order against David Smith, a former executive at Optum Health, from joining the venture, which is still unnamed but is often referred…