Martin Leibowitz, a veteran Wall Street research analyst, has left the board of Harvard Management Co., making him the sixth outside director overseeing the $37.6 billion university endowment to exit since the beginning of 2015. Leibowitz, who joined Morgan Stanley in equity research in 2004 after working for Salomon Brothers and money manager TIAA, left because he reached his term limit, Paul Finnegan, chairman of HMC’s board, said in an e-mail. Glenn Hutchins, co-founder of private equity firm Silver Lake Partners, left the board for the same reason earlier this year.