“The announcement did not directly address Mr. Levine’s history of health problems, but said that ‘faced with the demands of rehearsing and performing two large-scale operas simultaneously this fall,’ he would concentrate his energies on Wagner’s ‘Tannhäuser,’ which opens on Oct. 8. The overlapping runs of two of Mr. Levine’s favorite operas were to have been a highlight of the Met’s 2015-16 season.”