President Vladimir V. Putin on Monday abruptly reduced the responsibilities of a senior adviser who last week issued a sweeping criticism of the Kremlin’s leadership and expressed deep misgivings about the direction in which Russia was headed. In a presidential decree released without further comment, Mr. Putin relieved the adviser, Andrei N. Illarionov, of his duties as Russia’s envoy to the Group of 8, comprising the world’s major industrialized nations and Russia.