test/ruby/test_enumerator.rb: remove capture_io that is no longer needed

The deprecation warning was disabled, and the code to check the warning
was removed at 996af2c, thus capture_io
is no longer needed.

test/ruby/test_enumerator.rb

@@ -69,9 +69,7 @@ def (o = Object.new).each
 
   def test_initialize
     assert_equal([1, 2, 3], @obj.to_enum(:foo, 1, 2, 3).to_a)
-    _, err = capture_io do
-      assert_equal([1, 2, 3], Enumerator.new(@obj, :foo, 1, 2, 3).to_a)
-    end
+    assert_equal([1, 2, 3], Enumerator.new(@obj, :foo, 1, 2, 3).to_a)
     assert_equal([1, 2, 3], Enumerator.new { |y| i = 0; loop { y << (i += 1) } }.take(3))
     assert_raise(ArgumentError) { Enumerator.new }