Saul and Gayfryd Steinberg are serious about this simplifying-their-lives thing. They’ve already sold their 34-room Park Avenue apartment for $37 million and consigned its furnishings to Sotheby’s for a May 26 auction. Now the Steinbergs have decided to part with their treasure trove of Old Masters and have asked dealer Richard Feigen, who got the Steinbergs started on the collection almost 20 years ago, to sell the paintings. Highlights of the collection include Rubens’s “Death of Adonis,” destined for a museum, Titian’s “Salome with the Head of John the Baptist” and Jan Bruegel the Elder’s “Christ Preaching at the Seaport.” Feigen says the whole lot could go for $65 million.