The building housing Massachusetts General Hospital's dermatology department at 50 Staniford St. has a new owner, according to a Suffolk County deed. Less than a week after acquiring the 315 on A apartment tower in Fort Point for $130 million, an affiliate of Chicago-based Equity Residential has sold off a parcel located at 50 and 60 Staniford St. in Boston to an affiliate of RREEF Property Trust for $123.3 million, according to a Suffolk County deed.