Boston’s Handel & Haydn Society was once little more than a struggling concert organization in a city jam packed with music and musicians. Then Mary Deissler came along. 24 years later, H&H, as it’s known, has become one of Boston’s premier musical organizations, and supports itself with help from a $3 million endowment. Deissler is stepping down from her position as H&H’s CEO this week, and she leaves an important legacy for her successor to follow.